The Carrion Lord
The Emperor of Mankind has sat immobile on the Golden Throne for 10,000 years. He is a rotting corpse held together by sheer psychic will and ancient technology. To sustain his psychic struggle against the Chaos Gods, he requires fuel.
The Black Ships
Every day, the Black Ships arrive at Terra with their hold full of Psykers (wizards/mutants). 1,000 souls are fed into the Golden Throne daily. Their life essence is burned out in seconds to keep the Emperor's lighthouse (The Astronomican) shining.
The Cost of Survival
Is it evil to sacrifice 365,000 innocent people a year? * Yes: It is monstrous. * No: Without it, the Imperium collapses, and trillions die to Daemons. This is the central moral horror of 40k.