From 1d4chan

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.

During 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.

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He was also mentioned in the novel Talon of Horus by the Inquisition, when they were interrogating Iskandar Khayon about his past deeds and the many aliases and titles he had attributed to him by Imperial forces. Khayon recognized a lot (but not all) of the places they mentioned, and mocked them for their lack of knowledge of these matters. Notably, he claimed that "Ygethmor" is not a name, but a title meaning "threader" (of the warp) in Cthonic, and that this title has been applied to several different people over the years. However, he does not deny that he held the title at some point, though how many Ygethmors there actually are is unknown.

It should be noted that absolutely everything Khayon says in Talon of Horus should be taken with a grain of salt, as we have no way of verifying some of his claims. However, in this particular instance he has no particular reason to lie. 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. He is, however, mentioned in the 4th edition Chaos Space Marines codex, where it implies he will be aware of attempts by the Officio Assassinorum to go after the other commanders, but won't stop them because he has a lot to gain if they succeed.

Famous members of the Traitor Legions
Originating from
the Canon:
Abaddon - Ahzek Ahriman - Argel Tal - Cypher - Doomrider
Eidolon - Erebus - Fabius Bile - Haarken Worldclaimer - Honsou - Horus Aximand
Iskandar Khayon - Kharn - Kor Phaeron - Lheorvine Ukris - Lucius
Lugft Huron - Luther - Madox - Maloghurst - Necrosius the Undying - Occam - Sevatar
Shon'tu - Svane Vulfbad - Talos - Telemachon Lyras - Typhus - Ygethmor - Zardu Layak - Zhufor
Originating from
the games:
Araghast the Pillager - Azariah Kyras - Bale - Crull - Eliphas The Inheritor
Firaeveus Carron - Kain - Nemeroth - Neroth - Sindri Myr - Varius