D&D 2024 DM's Toolbox: On-the-Fly Rules A-Z

Introduction

Chapter 3 of the 2024 Dungeon Master's Guide is a reference chapter, not a read-front-to-back one - roughly thirty sub-rules covering everything from alignment to traps, organized alphabetically so you can find the one you need mid-session. This guide keeps that structure: every topic below gets its own section, sized to how often you'll actually reach for it at the table.

For the session-mechanics rules this chapter assumes you already know (D20 Tests, DCs, running combat and social interaction), see the Running the Game guide. Return to the Dungeon Master Guides hub for the full chapter map.

Alignment

Alignment is a roleplaying tool, not a constraint: a character's actions indicate their alignment rather than the other way around, and it's fine for players to drift or outright change their character's alignment as the story develops. It helps in three places - describing a player character's moral compass, sketching a creature's starting attitude in an encounter (a Chaotic Evil monster leans Hostile, a Lawful Good one leans Friendly), and summarizing an organization's ethos, which doesn't have to match the alignment of its individual members or even its leaders.

Chases

Standard combat movement makes chases dull - faster creatures always catch up, equal-Speed creatures never do. Instead:

• Starting a chase: roll Initiative for anyone not already in it, track the distance between quarry and pursuers, and designate the closest pursuer as the lead.
• Dashing: a participant can Dash a number of times equal to 3 plus its Constitution modifier (minimum once). Each extra Dash beyond that forces a DC 10 Constitution save or 1 Exhaustion level. A participant with 0 Speed drops out.
• Ending the chase: on Initiative count 0 each round, the quarry makes a Dexterity (Stealth) check (auto-fail if never out of the lead pursuer's sight) against the highest Passive Perception among the pursuers. Environmental factors like a crowded area or good hiding spots grant Advantage or Disadvantage on that check.
• Complications: roll 1d12 for each participant at the end of their turn on the urban or wilderness Chase Complications table; a result affects the next participant in Initiative order, not the roller.

Creating a Background

To build a custom background: choose three fitting abilities (Strength/Dexterity for physical work, Constitution for endurance, Intelligence/Wisdom for cerebral or spiritual pursuits, Charisma for performance or social work), one Origin feat, two skill proficiencies (no required link to the ability scores), one tool proficiency tied to the background's practice, and a starting-equipment package worth 50 GP that excludes Martial weapons and armor.

Creating a Creature

You can reskin a monster's superficial details freely: size and creature type, Intelligence/Wisdom/Charisma (leave Strength/Dexterity/Constitution alone unless you want to touch Challenge Rating), languages, skill proficiencies and Expertise, senses (Blindsight, Darkvision, Tremorsense, Truesight - none affect CR), spells (swap for a same-level spell, keep damage-dealing and non-damage spells in their own lanes), attack names and damage types, and up to one or two Resistances or Immunities if it has none already. You can also bolt on flavor traits (Aversion to Fire, Battle Ready, Fey Ancestry, Siege Monster, and similar) as long as they don't touch Hit Points, Temporary Hit Points, or the amount of damage the creature deals.

Creating a Magic Item

Modifying an existing item: swap a similar capability (a Potion of Climbing becomes a Potion of Swimming), change its form while keeping its properties (a Ring of the Ram becomes a wand), change its damage type (a Flame Tongue deals Lightning instead of Fire), or combine two same-rarity items into one (as long as no more than one required Attunement).

Building from scratch: an item should let a character do something new or improve something they can already do, and the simpler the activation the better. Use the Magic Item Power by Rarity table as a ceiling: Common items cap around a level 1 spell effect or no static bonus; Legendary items can reach a level 9 spell effect or a +4 bonus. Require Attunement whenever passing the item around the party would be disruptive, or when its bonus would stack badly with similar items.

Creating a Spell

Give the spell a unique name, check that it isn't so strong a caster would use it every single fight, make sure it fits the identity of who casts it (Sorcerers and Wizards don't typically heal), and avoid spells so narrow almost nobody will bother preparing them. For damage, use the Spell Damage table as a guide - roughly 1d10 per spell level against a single target at cantrip-to-level-9 scale, scaling up for multiple targets, assuming half damage on a successful save; bump damage by about 25% if your spell doesn't allow that half-damage save. The same table doubles as a healing-spell benchmark, except a cantrip shouldn't restore Hit Points at all.

Curses and Magical Contagions

Curses typically fall into a few buckets: Bestow Curse-tier curses (use the spell's 1-minute-to-until-dispelled range as a level 3-to-9 potency benchmark), curses tied to a specific creature's nature (a mummy's curse, a werewolf's bite), cursed magic items, environmental curses that afflict anyone who lingers in a tainted place, and narrative curses you build yourself for breaking a taboo - these should feel rare and tied to your campaign's lore, and the affected creature should be able to learn how to end it, often by righting the wrong they committed.

Magical contagions are diseases with a magical origin. All of them follow the same shape: an incubation period, an ongoing effect while infected, a periodic saving throw to fight it off (recuperating 5 days without interrupting a Long Rest grants Advantage on that save for the next 24 hours), and a way it spreads to other creatures. Published examples include Cackle Fever (Exhaustion and uncontrollable laughing fits), Sewer Plague (worsening Exhaustion from filth exposure), and Sight Rot (Blindness from tainted water, curable with Heal, Lesser Restoration, or a crafted ointment).

Death

Character death is emotionally loaded, so talk about how your table wants to handle it during Session Zero (see the Running the Game guide for that conversation). Keep it fair: don't cheat in the monsters' favor, don't make an encounter a personal punishment, warn players before a clearly fatal choice, and don't spring encounters they had no way to know they couldn't survive or escape.

Defeated, not dead is an optional alternative: a character who'd otherwise die instead drops to 1 Hit Point with the Unconscious condition, can still heal normally, but stays Unconscious until targeted by Greater Restoration or until they pass a DC 20 Constitution save after a Long Rest ("sudden awakening").

If the whole party dies (a TPK), that's a pivot point, not necessarily a game-ender: a fresh start with new characters, a council of deities the party must convince to return them, an escape from the underworld, imprisonment by their killers, being raised by a stranger (possibly decades later), or a rescue mission run with temporary characters are all built-in options the book suggests.

Doors

For bigger doors, double or triple the Hit Points and add 5 to the DC. A barred door opens from the barred side with a Utilize action to lift the bar. A locked door takes Thieves' Tools and a Dexterity (Sleight of Hand) check against a DC set by lock quality (Inferior DC 10, Good DC 15, Superior DC 20), after 1 action for a Simple lock or 1 minute for a Complex one. A secret door is found with the Search action and a Wisdom (Perception) check (DC 10-20 depending on how well it's hidden) or an Intelligence (Investigation) check if the clue is finding evidence a door exists rather than spotting the door itself - don't gate essential treasure or progress behind one unless you're fine with players never finding it. A portcullis gives creatures near it Three-Quarters Cover from the opposite side and takes a Utilize action plus a Strength (Athletics) check (DC 15-30 by size and material) to winch without access to the mechanism.

Dungeons

Roll or pick from the Dungeon Quirks table (a d100 table of origin stories: abandoned strongholds, cursed sites, treasure vaults, beholder-carved caverns, planar-portal ruins, and dozens more) to give a dungeon a distinctive identity beyond "rooms full of monsters," reflecting who built it, why, and what happened to them.

Environmental Effects

Planar effects bleed into the Material Plane near planar influence: Acheronian Bloodlust grants Temporary Hit Points for reducing a foe to 0 HP, Arcadian Vitality grants immunity to Frightened and Poisoned, Blessed Beneficence from Mount Celestia grants a passive Bless plus free Lesser Restoration on a Long Rest, Gehennan Cruelty can cause healing spells to fail outright, and the Winds of Pandemonium (including parts of the Underdark) pile on Exhaustion that a Long Rest alone can't clear. A dead magic zone works like a permanent Antimagic Field up to 300 feet across; a wild magic zone rolls a 1-in-20 chance of a Wild Magic Surge every time a spell slot is spent inside it.

Fear and Mental Stress

Adventurers are hardier than ordinary folk against fear, but the right creature or effect can still fray even a stalwart party's nerves - agree on using these rules with your table up front, the same conversation as any other sensitive-content check-in. When an effect causes intense mental stress, model it with Psychic damage; the Sample Mental Stress Effects table gives DCs and damage by severity, usually resisted with a Wisdom save (Intelligence or Charisma sometimes fit better), with a successful save optionally reducing damage rather than negating it outright. Repeated or severe stress can leave a prolonged effect behind even after the immediate danger passes - a lasting phobia, a compulsion, or a similar complication you design to fit the trigger.

Firearms and Explosives

The Firearms table covers modern and futuristic weapons (pistols, rifles, grenades, grenade launchers) for campaigns that mix them in - if you sell them, price modern firearms as Rare magic items and futuristic ones as Very Rare. Grenades explode in a 20-foot-radius Sphere when thrown up to 60 feet or launched up to 1,000 feet from a Grenade Launcher; a Fragmentation Grenade forces a DC 15 Dexterity save for 5d6 Piercing damage (half on a success), a Smoke Grenade Heavily Obscures the area for 1 minute. Gunpowder explodes if set alight: a DC 12 Dexterity save against 3d6 Fire damage from a powder horn or 7d6 from a full keg. Alien technology a party finds can be identified with an Intelligence (Investigation) check, DC 10 for something simple like a lighter, DC 20 for something complex like a computer.

Gods and Other Powers

Deities are ranked by cosmic scope: greater deities are ancient, near-incomprehensible forces behind major domains (the sun, death, agriculture) who can take any form they like; lesser deities govern narrower provinces and manifest more directly; quasi-deities (demigods, titans, vestiges) have divine origin but don't answer prayers the way full gods do. Gods aren't bound by mortal alignment categories, though most reside on an Outer Plane whose tone matches their general bent, and worshippers don't have to share a god's exact alignment to venerate them. Divine magic (Cleric, Druid, Paladin, and Ranger spellcasting) is mediated through gods and god-like forces, but the power itself isn't contingent on a character's day-to-day faith once granted. When a spell like Commune reaches out for divine knowledge, gods aren't omniscient, but they're deeply informed within their domain, and can predict the near future reliably. Divine intervention (blessings, emissaries, or outright miracles) should never guarantee a win or an automatic loss - it shifts the odds, not the outcome. Building your own pantheon can be as simple as one deity per Outer Plane (except the Abyss and Nine Hells, which belong to demons and Asmodeus respectively), or as sparse as a single god, a good/evil pair, or impersonal spirits and philosophies instead.

Hazards

Hazards are tagged nuisance or deadly for a specific level range - a nuisance hazard one range up can be deadly to a party one range down, so check the tag before dropping one on low-level characters. They're presented alphabetically in the book (Brown Mold, poisonous gas pockets, Yellow Mold, and dozens more), each with a trigger and effect; a representative example is Brown Mold, which chills the air within 30 feet and deals Cold damage (DC 12 Constitution save) to anyone who lingers within 5 feet of it.

Marks of Prestige

Beyond gold and magic items, adventurers earn less tangible rewards that shape how NPCs and factions treat them long after the gold's spent: fortifications granted by a political figure (with maintenance costs by size - 50 GP/day for an outpost up to 400 GP/day for a large castle - distinct from a Bastion, since the land stays crown property and can be revoked); letters of recommendation that open doors to NPCs a party couldn't otherwise reach, worth only as much as the letter-writer's reputation; medals with symbolic rather than mechanical value that shift how NPCs treat the wearer; titles, often paired with a land grant, inheritable in a feudal setting and revocable if the holder doesn't live up to them; and special training, which costs 50 days with a trainer in exchange for a skill proficiency, tool proficiency, or language.

Mobs

Speed up fights against large groups of identical monsters: use average damage from the stat block instead of rolling, treat a monster reduced to a handful of Hit Points as taken out of the fight, and split more than eight or so identical monsters into mobs of five to eight, spreading their turns between the party's turns (no more mobs than there are player characters). For a whole mob's D20 Test, find the roll needed to succeed (target number minus the monster's bonus), look it up on the Mob Results table (using the Advantage or Disadvantage column if it applies to the whole mob), and read off a fraction of the mob that succeeds instead of rolling individually. The Targets in Area of Effect table gives a rough headcount for how many monsters a Fireball or similar effect catches, adjustable by 1d3 if the mob is spread out or bunched together.

Nonplayer Characters

Most NPCs need only a couple of memorable details - a distinguishing tattoo, a broken nose - not a full write-up. For NPCs who'll play a real role, flesh out six elements: name (roll from the accompanying given-name/surname tables, or mix and match), stat block (pulled from the Monster Manual, customized with the "Creating a Creature" guidelines above if needed - skip this if the NPC won't fight or use special abilities), alignment, appearance, personality, and a secret (the NPC Secrets table offers seeds: hidden identity, a crime in progress, a threatened family member, a magical compulsion, financial ruin, unrequited passion). Recurring NPCs who show up again across a campaign, at higher-tier stat blocks as they and the party both grow, make a world feel lived-in - just don't over-invest in an NPC the party will only meet for three minutes.

Poison

Four delivery types: contact (smeared on an object, affects exposed skin on touch), ingested (a full dose in food or drink, with a partial dose optionally granting Advantage or half damage at your discretion), inhaled (affects everyone in a 5-foot cube on release - holding your breath doesn't help, since it affects nasal membranes and other exposed tissue directly), and injury (applied to a weapon or piece of ammunition as a Bonus Action, delivered through Piercing or Slashing damage). A character can attempt to harvest poison from a dead or Incapacitated venomous creature with a Poisoner's Kit and a DC 20 Intelligence (Nature) check, risking exposure to the poison on a bad enough failure. Published examples range from Assassin's Blood (150 GP, DC 10 Constitution save) to Purple Worm Poison (2,000 GP, DC 21 Constitution save for 10d6 Poison damage).

Renown

An optional rule for tracking a character's (or party's) standing within a faction, organization, or community, separate from character level. Use it campaign-wide (joining and climbing ranks in a guild) or adventure-scale (needing Renown 5+ with a local council before they'll share resources). Renown thresholds and their in-fiction meaning are yours to define per organization.

Settlements

A settlement doesn't need a map unless the party needs to know precisely where things are relative to each other - describing it is usually enough. The Settlement Tracker tables help flesh one out fast: a Defining Traits table (fortified walls, canals and bridges, ancient ruins, an impressive central structure) and a Claim to Fame table for what the place is known for.

Siege Equipment

Large or Medium objects with their own AC, Hit Points, and attack routines, usually requiring a crew to load, aim, and fire:

Loading and aiming each take their own Utilize action before a crew member can fire.

Traps

Traps are presented alphabetically with a Trigger, a Duration, an effect, and Detect and Disarm instructions, then scale up with a table of higher-level damage and DCs. A Collapsing Roof trap (deadly, levels 1-4) triggers on a trip wire, forcing a DC 13 Dexterity save against 2d10 Bludgeoning damage from falling debris, detected with a DC 11 Wisdom (Perception) check that reveals the trip wire and unstable ceiling. A Falling Net trap (nuisance, levels 1-4) restrains a target on a failed DC 10 Dexterity save, is detected with a DC 11 Wisdom (Perception) check, and frees a trapped creature with a DC 10 Strength (Athletics) check or by destroying the Net. Traps built this way follow the same structure you can reuse to design your own: a clear trigger, a proportionate effect for the intended level range, and a fair way to detect and disarm it before it goes off.

Related Guides

For the session mechanics this toolbox assumes (D20 Tests, DCs, running combat and social scenes), see the Running the Game guide. For building these ingredients into full encounters and adventures, see the Creating Adventures guide. Return to the Dungeon Master Guides hub for the full chapter map.