Umbral Horror Will o' the Wisp

The monster effect "When this card is Normal or Special Summoned: You can target 1 'Umbral' monster in your Graveyard or on your side of the field, except this card; this card's Level becomes the same as that monster's" is a Trigger Effect. (It targets 1 "Umbral" monster face-up in your Monster Zone other than "Umbral Will-o'-the-Wisp", or 1 "Umbral" monster in your Graveyard.)

The monster effect "this card's Level becomes the same as that monster's" is an optional effect that can be activated when "Umbral Will-o'-the-Wisp" itself is Normal or Special Summoned face-up. (It can also be activated during the Damage Step.)

The monster effect "When this face-up Attack Position card is destroyed by battle and sent to the Graveyard: Destroy the monster that destroyed this card" is a Trigger Effect that activates in the Graveyard. (It does not target.)

The monster effect "Destroy the monster that destroyed this card" is a mandatory effect that activates at the end of the Damage Step when "Umbral Will-o'-the-Wisp" itself, while face-up in Attack Position in the Monster Zone, is destroyed by battle and sent to the Graveyard.