Clan Verminus is a new Great Clan unique to Age of Sigmar. As the Great Clans in AoS are instead a type of Clan (So a plague-infested monk Clan will be a Pestilens clan among many), the "Clans Verminus" is essentially the old Warlord Clans. They're all Skavenslaves, Clanrats, Stormvermin and Warlords; none of the fancy shit. Then again, if you can't win by throwing thousands upon thousands of warm, ratty bodies at the enemy, how can you say you're playing Skaven?
Lore wise, they’re described as the most numerous, most aggressive, most territorial, and most fractious of the Great Clans. Its many warbands are always fighting amongst each other constantly. They worship the Great Horned Rat as the multi tailed King of Lashes, a conquering ruler who rewards only the ones who successfully slaughter his enemies.
Where Did They Come From?
Not surprisingly, GeeDubs offers little on how exactly the Clans Verminus originated. So we are left to speculate and ponder for ourselves. Currently the two most prevalent theories say that the Clans Verminus are a Clan Mors 2.0 or the combined form of all Old World Warlord Clans.
The Mors Ascended theory mostly stems from an interesting line from the Games Workshop webstore. During the limited rerelease of the original Skaven Hero miniatures, Queek Headtaker’s bio casually states that Clan Mors is a forefather of the Clans Verminus. It’s also not too much of a stretch to imagine the Great Horned Rat would reward Mors for their successes during the End Times by promoting them to rank of Great Clan and rebranding them as well. The only flaw with this theory is the existence of Clan Morskrit. A Verminus Clan that not only shares a similar name, but also wears the same blood red armor and is described as “ancient...tenacious survivors”. This could be some splinter group of Mors who wanted to keep the older traditions alive, but considering the very nature of Skaven, it’s unlikely.
Ironically the flaw in the previous theory actually helps to support the Warlord Collective Theory. Like Morskrit, there are several Verminus clans that directly reference clans of the Old World; Clan Scour wear the bones of their enemies as armor like the undead obsessed Clan Mordkin, and Clan Koniptik has patches of red fur like Clan Volkn. As to why a race of self centered warmongering megalomaniacs would decide to come together, well it could very easily be an “enemy of my enemy” situation. Seeing the Thrall Clans submit to their masters, the Warlords decided to pull their forces together so they’d all stand a chance if the other guys tried to make a move.