Losing hit points measures not just physical injury but also the target’s mental focus, positioning, armor damage, overall luck, and “plot armor” in the encounter. A “hit” may lower the target’s h.p. but be described as cutting the straps on the target’s plate mail, or causing a beast to stumble and pause a second (without literally invoking rules conditions or movement).
As a rule of thumb…
* 100% h.p. = “Uninjured”
* 75% to 99% h.p. = “Minimal wounds, bruising, normal up armor, stable focus”
* 50% to 74% h.p. = “Light wounds, minor bleeding, scratched up armor, focused”
* 25% to 50% h.p. = “Serious wounds, bloodied, dented armor, minor shock”
* 1 to 24% h.p. = “Critical wounds, very bloody, damaged armor, major shock”
As a reaction, a character can make a DC 15 Intelligence (Medicine) roll to tell exactly how many hit points a creature has left after its first injury.
From the optional rule listed in the Dungeon Master’s Guide (5e), p. 273:
A creature might flee under any of the following circumstances:
* The creature is surprised.
* The creature is reduced to half its hit points or fewer for the first time in the battle.
* The creature has no way to harm the opposing side on its turn.
A group of creatures might flee under any of the following circumstances:
* All the creatures in the group are surprised.
* The group’s leader is reduced to 0 hit points, incapacitated, taken prisoner, or removed from battle.
* The group is reduced to half its original size with no losses on the opposing side.
To determine whether a creature or group of creatures flees, make a DC 10 Wisdom saving throw for the creature or the group’s leader. If the opposition is overwhelming, the saving throw is made with disadvantage, or you can decide that the save fails automatically. If a group’s leader can’t make the saving throw for whatever reason, have the creature in the group with the next highest Charisma score make the saving throw instead.
On a failed save, the affected creature or group flees by the most expeditious route. If escape is impossible, the creature or group surrenders. If a creature or group that surrenders is attacked by its conquerors, the battle might resume, and it’s unlikely that further attempts to flee or surrender will be made.
A failed saving throw isn’t always to the adventurers’ benefit. For example, an ogre that flees from combat might put the rest of the dungeon on alert or run off with treasure that the characters had hoped to plunder.
Creatures at 0 hit points are unconscious and dying like the rules for player characters, but NPCs automatically fail at death saves each round. In other words, final “death” happens on the third round after dropping to 0 h.p. or if the fallen takes new damage while down. (This rule is meant to give PC time to cast healing magic on fallen allies or enemies sought for capture.) Ignore this rule for constructs.
CLEAVING THROUGH CREATURES
From the optional rule listed in the Dungeon Master’s Guide (5e), p. 272:
When a melee attack reduces an undamaged creature to 0 hit points, any excess damage from that attack might carry over to another creature nearby. The attacker targets another creature within reach and, if the original attack roll can hit it, applies any remaining damage to it. If that creature was undamaged and is likewise reduced to 0 hit points, repeat this process, carrying over the remaining damage until there are no valid targets, or until the damage carried over fails to reduce an undamaged creature to 0 hit points.
(This rule, loosely based on the 3.5e Cleave feat and AD&D1e fighter rules, allows a powerful warrior to chop through waves of weak enemies without having to roll attacks again. This change is more for cinematic flavor and speeding up combat encounters than game balance or realism.)
Per the rules, this action uses up a character’s reaction to act, but in some situations, I tend to allow the response to happen as a free action if meant as simply coordinating with an ally on the same maneuver. (Initiative order should not stop experienced characters from executing practiced tactics together.)
This rule from D&D 3.5 and Pathfinder allows characters to willingly lower their initiative order number. I tend to allow it.
SPECIALIZED SPELL LISTS
I tend to allow characters to add spells from other classes to their character’s available spell list if it is justified in a character’s backstory or background. For example, an archeologist-based cleric, paladin, ranger, sorcerer, or warlock character can learn the Identify spell even though that spell does not normally fall on those classes’ spell lists. This house rule does not add the Ritual Caster ability if the base class does not normally have it, though taking the feat of the same name fixes that limitation.