Cherubael

In the Warhammer 40,000 universe Cherubael, or the Cherub of Bael, is a daemonhost from the Eisenhorn trilogy by Dan Abnett. Cherubael was formerly in the service of Inquisitor Quixos, a renegade Inquisitor killed by Gregor Eisenhorn. Although Eisenhorn despises Cherubael at first, he begins to find need for the daemonhost's extraordinary power leading to Eisenhorn's slide into radicalism.

Origins
Cherubael was once a daemon-prince on the feral world Clanar II, where he dominated the primitive locals who came to worship him as a god. Quixos defeated Cherubael and enslaved him in the body of one of the warriors of Clanar II. Cherubael served Quixos until his death at the hands of Eisenhorn.

Dealings with Eisenhorn
Cherubael was able to discern the future from the Warp, and saw that Eisenhorn, if allowed, would destroy his physical body and so set Cherubael free. For over a century, Cherubael manipulated and protected Eisenhorn so that Eisenhorn would start down the path of radicalism and so set in motion the events that would free Cherubael. Eisenhorn, aided by a Force Rod crafted by Magos Geard Bure, finally destroyed Cherubael's host body on Farness Beta and banished him to the Immaterium. He also annihilated Cherubael's companion, another deamonhost known as Phrophaniti. However, during his conflicts with the heretic Fayde Thuring and his Corrupted Titan, The Cruor Vult, Eisenhorn found himself sorely pressed and summoned Cherubael, whom he had bound into a new host after defeating and banishing it. Eisenhorn came to rely heavily on Cherubael in the coming years, much to the dismay and anger of his staff and fellow Inquisitors.