Complex sentence: The founder of the Imperium is an enigmatic and mysterious persona known only as "the Emperor of Mankind", is a human psyker of phenomenal power, an immortal who has guided humanity since the dawn of civilization on Earth.
Split into simpler sentences: The founder of the Imperium is an enigmatic and mysterious persona known only as "the Emperor of Mankind". He is a human psyker of phenomenal power, an immortal who has guided humanity since the dawn of civilization on Earth.