Vorth was the sole survivor of a Grey Knights strike force on the fortress world of Mortain. They had stopped a daemonic rite by the chaos warband called The Company of the Shadow, only to be bombed by the Red Corsairs who thought the Company were still on the planet (although killing Grey Knights probably scored them big points with the chaos gods anyway).
Vorth was the only survivor and he suffered intense survivor's guilt afterwards, haunted by the voices of his slain men that would not lessen. He feared he had been corrupted but that was not the case; on another world, when he was separated from his comrades and about to become daemon chow, a force of Grey Knights appeared and saved him, leaving him confused over who his saviors were. It was only later, when he got a good look at his mysterious allies, that he finally knew who they were: the brothers who had been slain by the Red Corsairs on Mortain, who in their moment of death had become bonded to Vorth.
So even in death the Grey Knights couldn't leave their bro to do all the work.
Vorth believe his brothers suffer from their untimely deaths and only by having the head of Huron Blackheart, lord of the Red Corsairs, on a platter can he put them finally to rest. This is no small task as Huron is a hard bastard and head of a warband that is quickly growing to rival the traitor legions of old.
Nonetheless, Vorth is a typical heroic Grey Knight and not subject to things like limits so he will track down Huron and a battle of epic proportions will take place, while several Imperial planets will get blown up (as normal).