Crumbling Cathedral
From Grimhaven Wiki
The Crumbling Cathedral, once a magnificent center of worship dedicated to the deity of light and hope, now stands in eerie silence, its towering spires and intricate stonework weathered by time and neglect. Forgotten by civilization, this sacred place has become a chilling reminder of what was lost. Old legends claim that those who dare to explore its decaying halls often disappear without a trace, leading to whispers of supernatural forces at play.
To gain access to the island you will need to find the NPC Otis O'Malley he will bring you there by boat, he is located on Lonely Island.
The Crumbling Cathedral is also where you can find the Exterminator Quest.