Keurse, the Ogdoadic Light — Ruling

Effetto 1

The effect that Special Summons this card and allows your opponent to Special Summon a monster from their Graveyard is an Ignition Effect that can be activated in the Graveyard. (Tribute 1 monster you control as the cost of activating this effect.)
When resolving this effect, Special Summon this card from the Graveyard. Afterward, your opponent chooses whether or not to Special Summon a monster from their Graveyard with its effects negated. (These actions do not occur simultaneously.)

The effect that Special Summons a Level 4 or lower "Abhyss" monster from your Graveyard is a Trigger Effect that can be activated on the field. (This effect targets 1 Level 4 or lower "Abhyss" monster in your Graveyard. It can also be activated during the Damage Step.)