Amontep II is an arid, crimson Imperial Forge World similar to Mars that Archmagos Dominus Belisarius Cawl discovered contained deposits of the arcane, anti-psychic material known as blackstone in the wake of the birth of the Great Rift. Cawl ordered one of his chief agents, Magos Dominus Dentrex Ologostion, to determine if the world contained large quantities of blackstone. Ologostion was dispatched with a force of Skitarii and Knights Armiger to locate, sample and secure the largest deposits of blackstone available.
When the Adeptus Mechanicus expeditionary force arrived at a large dust bowl near the Vrackian Megacomplex, they discovered in the course of their excavations that the blackstone was part of ancient Necron ruins -- Amontep II was actually a Tomb World of the lost Necron Empire. Their actions awakened the sleeping Necrons who assaulted the Mechanicus troops.
At the same time, what the servants of the Machine God had assumed to be ancient ruins were soon revealed following their activation to be fully functional Necron Obelisks. Recognising that his forces were overwhelmed, Ologostion focused instead on seizing a sample of blackstone for Cawl from the existing Necron structures constructed from the material and then escaped off-world.
The Mechanicus vessels in orbit of Amontep II unleashed their firepower on the Necron structures near the Vrackian dust bowl, destroying them utterly. Bu the narrow victory won by the Adeptus Mechanicus over their Necron competitors for blackstone would bear a terrible cost -- with its noctilith structures lost or damaged, Amontep II was left vulnerable to Warp incursions. Soon after the battle, in the skies high above its desert surface, a livid yellow Warp rift began to shimmer, whirl and grow ever larger...
- Codex: Necrons (8th Edition), "A New Epoch: Foundation of Sand," pg. 31
- Warhammer 40,000: Forgebane (8th Edition) (Boxed Set Book), pp. 18-23