Grotesque Guardians
The Grotesque Guardians are a pair of gargoyles, Dusk and Dawn, found on the Slayer Tower's rooftop, whose entrance is found on the 3rd Floor. In order to access the roof, players must obtain a brittle key from gargoyles while assigned them for a Slayer task (requiring 75 Slayer).
Similar to other Slayer bosses, such as Cerberus and the Abyssal Sire, the Grotesque Guardians can only be killed while the player is assigned gargoyles or the Guardians themselves as a slayer task.
Despite Dusk and Dawn being two different entities, killing both guardians only counts as one kill due to the Dusk absorbing Dawn's essence. However, if the player has the Double Trouble perk unlocked, Dawn's death will also be included, although Dusk still needs to be killed and the full boss fight completed in order to receive experience and task progression.