He rules over cackling, pink-limbed horrors and oath-sworn warriors clad in armour of amethyst and lapis lazuli and wielding ensorcelled blades. The silvery glaive he wields was a gift from one daemon that was his acquaintance and it is as light as one of her feathers. 
He is tall and slim of build with gold and azure armour that seems moulded to his body - his helm that of a swooping bird, the visor a frowning face. Three small books and a slughorn are chained to his belt whilst streamers of various shades of blue dangle from his battle-plate. Until he encountered Carkus, his armour and body were tainted by patches of mould, which he was forced to constantly tear or burn away but which was slowly killing him. 
Long ago, he was a knight of what is now known as the Blighted Duchies in the Realm of Ghyran and was present when the Grandmaster of his order drank from the Flyblown Chalice at the behest of the Lady of Cankerwall to defeat time in service to Nurgle. He and his brothers in arms fought for her love and he remains bitter towards her. 
In the Stalking Keep Ompallious attacked a tourney being held to choose the next duke of Festerfane and abducted the Lady of Cankerwall, retreating to the depths of Bullawood. He was pursued by two score and ten knights of the Order of the Fly as well as Carkus Gryme, but all but the young heir to the duchy were slain by the Keep and Mordrek the Damned who Ompallious had brought back from the ruin of the World-that-Was. 
When Carkus achieved the impossible and defeated Mordrek, Ompallious granted him an audience, even as he held the lady captive until she bargained for the life of the young heir, offering to remove the curse of Bollos which was killing him. He agreed, but would have struck down Carkus if Mordrek had not intervened on his behalf. The Radiant Knight once more, free of the curse, departed in his keep. 
|You gave up so much, for vengeance. You let the Great Changer hollow you out of all but that singular ambition. But your end is here, and there is love yet in the ending. And serenity in acceptance. Come back to me, my love - my sweetest knight...
~The Lady of Cankerwall to Ompallious Zeyros.[1a]