The Legio Infernus is a splinter group of the Black Legion. Following Abaddon into exile, they broke away when the Despoiler spat on the Heritage of Horus and declared himself Warmaster. Khyron, by that time Lodge Master, led the malcontents deeper into the Eye of Terror, where they finally found a world to claim for their own.
They made pacts with the Dark Powers and eventually grew into a dangerous force. Allying themselves with powers outside the Eye of Terror, they secured an industrial base and began to harass imperial and eldar forces and, of course, enemy chaos forces.
Khyron earned the enmity of many powerful individuals, chief among them the nurglesque warlord Ebolai and imperial Grand Inquisitor Tannhäuser, who thwarted the chaos lord's efforts many times.
Khyron, however, eventually was granted the rank of Daemon Prince and finally, even Abaddon had to recognize his power and ability. The newly-born daemon prince was one of the three individuals who were responsible of forming the League of Chaos. It is rumored, however, that it was Abaddon's plan to embarass Khyron. The League fought well in the 13th crusade, but many members eventually broke away to pursue their own goals, leaving Khyron's flanks open and he had to consolidate, allowing imperial reinforcements flowing into Belis Corona.
Nowadays, the Daemon Prince continues to build his forces and consolidates his gains. He is aware that he cannot hold the planets they captured forever, so he exploits their every resource to the limit.

The Legio leans heavily towards the use of combined arms. Sorceres of Tzeentch find in the Legio Infernus kindred spirits for their use and worship of fire and the titans of the Legio Vulkanum, whose survivors fled into the eye, are often found in Khyron's serivce.