Is Horus Lupercal Dead?

The sacrifice of the Custodian bought the Emperor time to deliver a finishing blow to Horus. With iron resolve, he gathered his full strength and delivered a massive psychic blow that killed Horus almost instantly and obliterated his very soul.

Is Horus really dead?

The Emperor utterly erased Horus from existence and was found dying by Rogal Dorn, primarch of the Imperial Fists. On the edge of death, the Emperor instructed Dorn to place him in the life support of the golden throne, where he has spent the last ten thousand years slowly decaying yet still alive.

Who killed Primarch Horus?

He was slain, some believe willingly, by the Callidus Assassin M'Shen on the world of Tsagualsa in the Eastern Fringe of the galaxy immediately following the end of the Horus Heresy in the early 31st Millennium.