Ygethmor

Ygethmor, also called Ygethmor the Deceiver, was a Black Legion Chaos Sorcerer and Chosen of Abaddon. His biggest claim to fame was being the big boss of the Chaos forces in the clusterfuck known as the Medusa V campaign, where his official goal was to claim the planet for Chaos, but he was really in it to become a Daemon Prince. On the way to Medusa V, two entire planets on the way turned to the Dark Gods in an orgy of bloodshed that left those two planets largely depopulated; it was speculated that this was some kind of psychic influence from Ygethmor, or a new mind-raping superweapon. Either way, it would have been a much handier tool for the Black Legion if Ygethmor lived long enough to share the secret.

Despite the massive clusterfuck of every faction fighting each other in a single war on a single planet, many Chaos Champions managed to ascend to daemonhood on the battlefields, though Ygethmor did not. Whether this was because of a Chaos-contractual obligation that he had to actually make a planetkill to get there, or because of his own incompetence and being outperformed by his underlings, is probably a matter best left to subjective interpretation. His own part of the campaign ended when the Eldar Autarch in charge of the Craftworld forces decapitated him personally, but she was immediately killed by one of his bodyguards in the next assault phase.

He was also mentioned in the novel Talon of Horus by the Inquisition, when they were interrogating Iskandor Khayon about his past deeds and the many aliases and titles he had attributed to him by Imperial forces. Khayon recognized a lot of the places they mentioned, but a couple he didn't, and he didn't recognize anybody by the name of Ygethmor (which was one of the names they were trying to pin him with). It's possible "Ygethmor the Deceiver" was an name later attributed to Khayon after the original's death, or simply a person or an alias he didn't recognize. Worst case scenario is that ADB might have disowned Ygethmor from the canon for losing that campaign so hard. Ouch.

He was mentioned in a few other places as a background reference, including in Black Crusade. He's the leader of the Black Legion's Tzeentch followers in the Black Legion supplement codex, where his death isn't mentioned. However, the battle of Medusa V took place at the very tail end of the 41st Millennium, so it might just not have happened yet in the codex.