Will-O'-Wisp

Will-O'-Wisp CR: 2

Tiny undead, chaotic evil

Armor Class: 19

Hit Points: 22 9d4

Speed: 0 ft , fly: 50 ft , can hover

STR

1 -5

DEX

28 +9

CON

10 +0

INT

13 +1

WIS

14 +2

CHA

11 +0

Damage Resistances: Acid, cold, fire, necrotic, thunder, bludgeoning, piercing, and slashing by nonmagical attacks

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 Rating: 2

Consume Life. As a bonus action, the will-o'-wisp one creature it can see 5 feet of it with 0 hit points and 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 are difficult terrain. It takes 5 1d10 force damage if it ends it's turn inside an object.
Variable Illumination. The will-o'-wisp sheds light in a 5 to 20 foot and dim light for an additional number of feet equal to the chosen radius. The will-o'-wisp can alter the radius as a bonus action.

Actions

Shock. Melee Weapon Attack +4 to hit, reach 5 ft, one creature. Hit 9 2d8 lightning damage.
Invisibility. The will-o'-wisp and it's light magically become invisible until it attacks or uses it's Consume Life, or until it's concentration ends (as if concentrating on a spell).

Will-O'-Wisps appear as glowing balls of light, usually in and around swamps, bogs, and brackish rivers & streams. They are the souls of the deceased that died in the water or swamp, and their hatred of dying by drowning drive them to try to drown others. This is done by them trying to get the living to follow them deeper into regions that are highly dangerous to life not adapted to the swamps and rivers they 'live' in.

Suggested Environments

Swamps, nogs, and rivers.