The main Grendel series. It followed the Grendel back-up story, entitled Devil by the Deed, in Mage: The Hero Discovered. The Grendel comics that immediately followed this series included multiple Grendel Tales miniseries as well as Grendel: War Child.
The preceding Devil by the Deed story was about Hunter Rose. This volume was split into multiple arcs, each of which showed a new step in the evolution of Grendel as it took over multiple individuals and eventually all of society. Many of these arcs have since been reprinted as collections, sometimes multiple times. The arcs are:
Devil's Legacy: Grendel 1-12, centered on Christine Spar
The Devil Inside: Grendel 13-15, centered on Brian Li Sung
Devil Tracks and Devil Eyes: Grendel 16-19, centered on flashbacks to the Hunter Rose era
The Incubation Years: Grendel 20-23, centered on Grendel's expansion from a series of individuals to all of society
God and the Devil: Grendel 24-33, centered on Eppy Thatcher, Innocent XLII, and Orion Assante
Devil's Reign: Grendel 34-40, centered on Orion Assante and a group of vampires in VEGAS
Matt Wagner wrote the entire series, using different artists for each story arc.
Grendel carried a "parental guidance suggested" label due to its extreme violence and sexuality, including homosexuality and incest. It often dealt with serious subject matter such as society and religion, often quite critically.