Xeiros Prime is a forbidden world in the Malfian Sub-sector of the Calixis Sector. During the rise of the arch-heretek Umbra Malygris, he and his followers subjected the worlds of Xeiros Prime, Loss and MMX215 to terrifying experiments. Orbital apostasic arrays drove entire planets into raving insanity. Bio-forged horrors stalked formerly-bountiful wastelands, transgenic atrocities ravaged entire cities, and enormous gene-lathes processed entire populations into ulcerous, writhing seas of living tissue. The infamous heretek was eventually stopped in a joint Inquisitorial-Mechanicus purge in the system of Synford II.