Our group plays over voice chat, with no battle map or visual aids, and my players can get confused when confronted with a large battlefield with many individual targets. To help large encounters run more smoothly, I devised the following rules for handling groups of enemies. They work best with melee or ranged fighters, especially hordes of goblins, kobolds and other low-level threats.
Forming a Mob
When four or more small, medium or large units with the same stats are within 5 feet of each other, they form a mob, and the following rules apply. Mob rules for targeting apply only to melee and ranged spell and martial attacks; area of effect spells, lair actions, and other effects are ajudicated as normal.
Mobs move as a single unit, acting on the highest initiative count rolled for any of its members. Mobs occupy 1 square for every 5 small creatures, 3 medium creatures or 1 large creature.
A mob counts as a single target. In cases where abilities specifically target another creature within 5 feet of the initial target, eg. hordebreaker, the ability may target the same mob.
A Mob’s AC is equal to that of any single unit; its HP is equal to that of a single unit times the number of units in the mob. On a successful melee or ranged attack, damage is applied to one unit at a time. If the total damage done to a mob reaches the HP of a single unit, the number of units in the mob is reduced by one. Example: a mob of five goblins with AC 10 and HP 35 (7 HP x 5 goblins) is hit by a melee attack for 10 damage. One goblin dies, reducing the mob size to 4, with 25 HP remaining. The mob is then hit with a ranged attack for 4 damage, reducing the mob size to 3 with an HP of 21.
Attack. On its turn, a mob may take the Attack action. It makes the number of melee or ranged attacks equal to the number of units remaining in the mob against a single target. If the mob units have multiattack, the mob may select a different target for each attack action. Example: a mob of 5 bandit captains engage with 2 PCs. Using their multiattack ability, they make 5 attacks against one PC, and 5 attacks against the second PC.
Pile On. If the mob makes a number of attacks less than or equal to the number of units in the mob minus 2, the mob can take the Pile On ability as a bonus action. The mob makes a Strength (Athletics) check with advantage, contested by either the target’s Strength (Athletics) or Dexterity (Acrobatics) check. If the mob succeeds, the target is grappled and knocked prone. The target may repeat the contested check as an action.
The mob gains a +1 on its Strength (Athletics) check for each unit above 2 that it devotes to the pile on action. While piling on, these units cannot attack on the mobs turn.