Tiny undead, chaotic evil
Damage Resistances acid, cold, fire, necrotic, thunder; bludgeoning, piercing, and slashing from nonmagical weapons
Damage Immunities lightning, poison
Condition Immunities exhaustion, grappled, paralyzed, poisoned, prone, restrained, unconscious
Senses darkvision 120 ft., passive Perception 12
Languages the languages it knew in life
Challenge 2 (450 XP) Proficiency Bonus +2
Traits
Consume Life. As a bonus action, the will-o'-wisp can target one creature it can see within 5 ft. of it that has 0 hit points and is still alive. The target must succeed on a DC 10 Constitution saving throw against this magic or die. If the target dies, the will-o'-wisp regains 10 (3d6) hit points.
Ephemeral. The will-o'-wisp can't wear or carry anything.
Incorporeal Movement. The will-o'-wisp can move through other creatures and objects as if they were difficult terrain. It takes 5 (1d10) force damage if it ends its turn inside an object.
Variable Illumination. The will-o'-wisp sheds bright light in a 5- to 20-foot radius and dim light for an additional number of ft. equal to the chosen radius. The will-o'-wisp can alter the radius as a bonus action.
Actions
Shock. Melee Spell Attack: +4 to hit, reach 5 ft., one creature. Hit: 9 (2d8) lightning damage.
Invisibility. The will-o'-wisp and its light magically become invisible until it attacks or uses its Consume Life, or until its concentration ends (as if concentrating on a spell).