Nobody said that Skaven clans can't be offshoots of other clans. For Clan Gritus, that's the start of their history. Long ago, they were part of Clan Mors, but the passing of one "Great Warlord Vrrmik" during the Skaven's failed attempt to restore control over the Empire of Man saw the oversized clan fracture. Since then, Gritus has made a name for itself- a clan that hunts other (always weaker) clans and either kills them or absorbs them. Because of this, a massive slave surplus has spawned (even when the cannibalism of weaklings and trading has finished), creating a second side to Gritus- master pit fighters (which does lead to slave revolts). Scars are present on all such combatants- heck, even the Warlord has a missing eye!
Interesting fact: Gritus still keeps the rags worn from their days in Clan Mors- red cloaks and the parent clan's markings still persist on older members.
The Hellbeast of Seep-Gore- Clan Gritus has become so successful in preying on weakling clans that, in recent years, they have amassed a large surplus of wealth. The clan's Chieftains have traded chests filled with warpstone and commissioned the Master Moulders of Hell Pit to create for them a hideous creature of unparalleled size and ferocity- the Hellbeast of Seep-Gore. With it, Clan Gritus plans to attack and slaughter its former master, Clan Mors; the time of revenge is almost at hand. Although no specific image of this beast exists, it may be a large version of the Hell Pit Abomination- the most unpredictable and dangerous of Clan Moulder's known creations.