Grimmbold lived in Sigil on Concordant Opposition, but the explosion of a portal sent him to Faerûn, near the Citadel of the Raven. He asked for help from a local and made a bargain with the Zhentarim: a major mission for them in exchange for a way to return to Sigil.
Grimmbold in 1369 DR was tasked with the mission of going to Skullport and leading a Zhent cell and discovering everything about the city, Halaster Blackcloak, and Undermountain, then killing the mad archmage and conquering the place. However, the githzerai and his minions had only vague information and soon most of the other members of the cell were slaughtered, devoured, incinerated, or disappeared without a trace. With only two companions, the half-ogre Jom Bovine and the rock gnome illusionist Esten, Grimmbold decided to abandon the mission.
Grimmbold hated Skullport and his inability to return to his home. Knowing his mission for the Zhents had been a suicide mission, he had no loyalty to them.