Toward the end (as I remember it) of the last Einsenhorn book, Gregor binds Cherubael to one of his warband ? Who was Cherubael bound to ? Was it Fischig ?
Yes, Cherubael gets bounced around a few times during the events of Hereticus, but he does indeed end up in Fischig by the end of the book.
At the start of Hereticus, Eisenhorn binds Cherubael to his (new) interrogator, Verveuk.
Later on, Aemos summons Cherubael into a random astropath from the Essene:
I looked at Aemos. 'You summoned Cherubael? It was you, wasn't it? You summoned him into into one of the Essene's poor astropaths?'
- Page 741, Eisenhorn Omnibus, by Dan Abnett
Later, after Fischig is killed, Aemos and Eisenhorn work rapidly to bind Cherubael to his corpse.
It was going to take a while to get used to Cherubaels new face, though. Once installed within the flesh host, the daemon had made its usual alterations - the sprouting nub horns, the talons, the smooth, glossy skin, the blank eyes.
But it had not entirely erased the features of Godwin Fischig.
- Page 752, Eisenhorn Omnibus, by Dan Abnett