Armor Class 19
Hit Points 22 (9d4)
Speed fly 50 ft.
Damage Resistances acid, cold, fire, necrotic, thunder, [object Object]
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
Consume Life. As a bonus action, the will-o'-wisp can target one creature it can see within 5 feet 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 3. 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.
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).
Monster Manual (2014), p. 301