Give any custom monster impactful features and attacks that make sense for their place in the game. When a monster feature deals damage, choose a damage type appropriate to the creature's physiology, theme, or story. A creature channeling magical power might deal acid, cold, fire, lightning, force, poison, psychic, necrotic, radiant, or thunder damage. A creature making use of spines, spikes, or projectiles might deal bludgeoning, piercing, or slashing damage.
Damaging Blast. This creature has one or more single-target ranged attacks using the attack bonus and damage calculated above, and which deal damage of an appropriate type.
Damage Reflection. Whenever a creature within 5 feet of this creature hits them with a melee attack, the attacker takes damage in return of a type appropriate to the creature. The damage dealt is equal to half the damage of one of this creature's attacks. If you give a creature this feature, give them one less attack than normal.
Misty Step. As a bonus action, this creature can teleport up to 30 feet to an unoccupied space they can see.
Knockdown. When this creature hits a target with a melee attack, the target must succeed on a Strength saving throw or be knocked prone.
Restraining Grab. When this creature hits a target with a melee attack, the target is grappled (escape DC based on this creature's Strength or Dexterity modifier). While grappled, the target is restrained.
Damaging Burst. As an action, this creature can create a burst of energy, magic, spines, or some other effect in a 10-foot-radius sphere, either around themself or at a point within 120 feet. Each creature in that area must make a Dexterity, Constitution, or Wisdom saving throw (your choice, based on the type of burst). On a failure, a target takes damage of an appropriate type equal to half this creature's total damage per round. On a success, a target takes half as much damage.
Cunning Action. On each of their turns, this creature can use a bonus action to take the Dash, Disengage, or Hide action.
Damaging Aura. Each creature who starts their turn within 10 feet of this creature takes damage of a type appropriate to the creature. The damage dealt is equal to half the damage of one of this creature's attacks. If you give a creature this feature, give them one less attack than normal.
Energy Weapons. The creature's weapon attacks deal extra damage of an appropriate type. You can add this damage on top of the creature's regular damage output to give them a combat boost, or you can replace some of the creature's normal weapon damage with this energy damage.
Damage Transference. When this creature takes damage, they can transfer half or all of that damage (your choice) to a willing creature within 30 or 60 feet of them. This feature is particularly good for boss monsters.
Creatures may inflict conditions on attacks. Here's a list of potential conditions. Be careful to ensure such conditions don't take away too much agency from the characters or their players. Aim towards fun.
Various challenges, obstacles, and magics can lead to either fatigue or strife. An effect can give a creature one or more levels of fatigue or strife (detailed in the effect's description).
If a creature suffering from fatigue or strife fails to resist another effect that causes a level of the tracked condition, its current level increases by the amount specified in the effect's description.
A creature suffers the effect of its current level in a tracked condition as well as all lower levels. For example, a creature suffering level 3 fatigue has its Speed halved, it cannot Sprint, and it makes Strength, Dexterity, and Constitution checks with disadvantage.
An effect that removes a tracked condition reduces its level as specified in the effect's description, with all tracked condition effects ending when a creature's condition level is reduced below 1.
Finishing a long rest reduces a creature's fatigue and strife levels by 1.
Fatigue represents exhaustion, exposure, hunger, injuries, and other physical factors which gradually wear a creature down. A creature which reaches the 7th level of the fatigue track becomes doomed and dies.
Fatigue Level Effects
Strife represents corruption, despair, fear, loss of resolve, and other mental factors which gradually undo a creature's very soul. A creature which reaches the 7th level of the strife track suffers a special, permanent effect, which is either randomly selected or decided by the GM. This might involve the creature shutting down completely, or being impacted in such a way that it is forever changed.
Strife Level Effects
This work includes material taken from the Lazy GM's 5e Monster Builder Resource Document written by Teos Abadía of Alphastream.org, Scott Fitzgerald Gray of Insaneangel.com, and Michael E. Shea of SlyFlourish.com, available under a Creative Commons Attribution 4.0 International License.
This work includes material taken from the A5E System Reference Document (A5ESRD) by EN Publishing and available at A5ESRD.com, based on Level Up: Advanced 5th Edition, available at www.levelup5e.com. The A5ESRD is licensed under the Creative Commons Attribution 4.0 International License available at https://creativecommons.org/licenses/by/4.0/legalcode.
This work includes material taken from the System Reference Document 5.1 ("SRD 5.1") by Wizards of the Coast LLC and available at https://dnd.wizards.com/resources/systems-reference-document. The SRD 5.1 is licensed under the Creative Commons Attribution 4.0 International License available at https://creativecommons.org/licenses/by/4.0/legalcode.