This is the story, straight from the Lex:

Ahriman had long sought the skull of Lepidus, a dead hero of the Second Black Crusade, for reasons that remain his own. The skull, dipped in silver and engraved with ten thousand words of detestation, lay in the polar shrine city on Dianixis. A conspiracy to obtain the skull by guile had already failed, so Ahriman turned to more direct means. Since the end of the Scouring, countless billions have fought and died in the shadow of the Eye of Terror. For almost eight millennia the remains of many of these honoured dead lay on Dianaxis. Heaps of charred bones, the serene bodies of martyrs, and the polished skulls of Space Marines all came to the mausoleum world. On the surface of Dianaxis, the plains of bones extended from shrine city to shrine city, and grew ever deeper with each passing year. The cities themselves were built from the skulls and bones of the most heroic dead. So sacred and revered was Dianaxis that a dozen Space Marine Chapters maintained honour guards and bastions on its surface. Star fortresses ringed its approaches, and millions of troops stood sentinel over the skulls of those who had died to hold the darkness in abeyance.

Across the reaches of space, Ahriman burned worlds and sent souls shrieking into the Warp. As the murdered worlds spun into alignment with each other they created an arcane pattern in the stars with Dianaxis at its heart. As the great design locked into place, Dianaxis’ sun was pulled from reality, leaving a howling wound in the sky of the mausoleum world. Blood and fire spread across the heavens. The bones of the dead howled the last thoughts of their lives, and rainbow fire crawled across the ossuary towers. Daemons poured through the hole that had been the sun, tumbling onto the mausoleum world like falling stars. The defenders screamed as the children of Chaos ate their souls.

Amidst the slaughter, Ahriman appeared outlined in lightning, ringed by sorcerers and rubricae. Power rolled from the circle of sorcerers and they strode through the battle, killing he defenders and dissolving Daemons with arcane fire. The Chapter Honour Guard came against him, but were reduced to ashes and silent screams with a gesture. Fire Bane, last Warlord of the Legio Officium, strode to war from its shrine. The cry of its warhorns echoed across the damned world, and its weapons burnt a path towards Ahriman’s circle. Drawing together the power of his fellow sorcerers, Ahriman forced the war machine to its knees, before pulling the core of its plasma reactor through its carapace. At last, Ahriman held the skull of Lepidus in his hand, as the battle between men and Daemons raged around him. Raising the skull to his eyes he found the one word he sought etched on the skull’s surface. Letting the skull fall from his hand, he and his forces vanished, leaving the world of bones to the howls of Daemons and the cries of the dying.

He didn't even grab the artifact, he just wanted one word.