The Barnacle's Revenge Campaign
Not much is known about Groteus, as he is not a major god of Golarion. From what you have found out, Groteus is the God of End Times and the Devourer of Worlds.
In Bloodcove, the Crew confronted the necromancer Petrune, who killed himself in a sacrifice to bring Golarion to the attention of Groteus. With the combined efforts of the Crew, they managed to close the portal through which Groteus could view the world, but were left marked by the experience.