Samus (Warhammer 40,000)
Near the end of the Great Crusade, the Luna Wolves pacified a planet catalogued as 63-19, known to its inhabitants as Terra. In the final stages of compliance, a Luna Wolves speartip went into the Whisperhead Mountains to eliminate a rogue force loyal to the planet's slain emperor. During the descent, the drop ships picked up a transmission that appeared intended to scare them, a repeating whisper of the following:
"Samus. That's the only name you'll hear. Samus. It means the end and the death. Samus. I am Samus. Samus is all around you. Samus is the man beside you. Samus will gnaw on your bones. Look out! Samus is here."
Then the Legionary Xayver Jubal succumbed to Samus's corruption and it possessed him, using him to attack the speartip. After a concerted effort, Garviel Loken killed the mutated Jubal and had his warped body destroyed. Horus ordered the suppression of the records of the encounter, especially Jubal's corruption, and denied that "Samus" was anything supernatural.
Samus becomes a recurring character in the Horus Heresy series. He is summoned by various traitor forces into various corpses at key battles like Calth and the Siege of Terra, and proceeds to die in every one of these appearances. Forge World gave him a model called "Samus, Daemon Prince of the Ruinstorm." Apparently he is a daemon prince of the Ruinstorm and may have killed Dorn had the duel continued any longer. Though the only one to make that claim was Samus himself, so it's hard to take that boast seriously.
The origin of Samus was finally revealed in the last of the Horus Heresy books: The End and the Death Part III. In the novel, Samus is found to be an echo in the warp of the betrayal of Garviel Loken, who died when FUCKING EREBUS stabbed him in the back with an anathame after Abaddon was going to let him leave alive. Erebus then explains that such a betrayal after all that the loyalist Luna Wolf had been though would resonate in the warp, allowing Samus to exist long before his inception. But we all really know Erebus did that just cause he's a cunt. And because we really really needed a Samus backstory apparently.
Interestingly Old Samus rules treated him like a Khorne demon, but later fluff suggests he's a greater daemon of the Dark King.
Upon attempting to fight Metroid's Samus for the right to be the one true Samus, the Daemon Prince was promptly pile-driven and then drop kicked so hard he has yet to do anything of importance since the Horus Heresy. As of the Siege Of Terra we've been getting hints that his disappearance might be the result of suffering a True Death in the final hours of the siege, but it's too soon to say for sure.
