Arthur the Rotten, also known as Pestilence is the champion of Vermin and the leader of the Vermin Order.
Background
Growing up as a peasant child on the outskirts of Durporden, Arthur was a believer in the Christian god, thinking as long as he was studious, hard working, and free of sin he'd make it into God's kingdom in Heaven.
Slowly as he grew older, he started to question his beliefs, and after a chance encounter with Vermin worshippers, during which he was infected with an illness, his belief fell to pieces.
He saw these rotbloods speak in tongues, conjure pests out of thin air, and spread disease with a flick of the wrist.
He locked himself in his room for two straight months trying to reclaim his faith from the oozing clutches of Vermin, but he couldn't.
He sought out the rotbloods that gave him the ailment, seeking to learn the power they held.
He became a devout rotblood in no time at all, and started roaming the Earth to spread Vermin's influence and disease across the world.
Just before the first plague, his actions and preaching put him in Vermin's sights.
Appearing to him in a swarm of cockroaches, Vermin told Arthur that he was her champion, The White Rider, The harbinger of the apocalypse, Pestilence.
Gathering his acolytes, he founded the Vermin Order with the sole mission to spread her disease all over the land and wipe out the true disease, humanity.