Karthus is a massive hellkite native to Jund, and a king among dragonkind. While all of Jund's dragons command the respect of the rest of the food chain below them, Karrthus commands the respect of his fellow dragons.
In the predator-filled land of Jund, Karrthus was the mightiest there was. Admired by Sarkhan Vol, Karrthus was captured by Nicol Bolas and forced into servitude. This desecration of Karrthus's power planted a seed of dissatisfaction in Sarkhan Vol, who started questioning his own commitment to Bolas.