During the Horus Heresy, Grendel served with the 16th Company of the 2nd Grand Battalion under Warsmith Berossus.

By M41 Grendel traveled as Captain of Arms in the retinue of Warsmith Berossus on Medrengard, during the latter's assault against the renegade Warsmith Honsou's fortress of Khalan-Ghol. Grendel watched as Honsou and his Possessed bodyguard, Onyx, tore open Berossus's Dreadnought carapace, and Honsou crushed the remains of the Warsmith's skull.