Will-o'-wisp

 are a type of spectral undead creature.

Appearance
Will-o'-wisps had a fading dull yellow-green glow and was surrounded by sparks.

Abilities
Resistant to acid, cold, necrotic, thunder, and nonmagical physical weapon damage.

Incorporeal Movement: The will-o’-wisp could fly through other creatures and Objects.

Shock: The will-o’-wisp could attack a target with this surrounding lightning.