Who is Leto II Atreides from God Emperor of Dune and what is their importance?

The God Emperor and antihero of this novel (and its predecessor, Children of Dune), Leto II is born over 3,000 years ago to Paul Muad'Dib Atreides and his Fremen consort Chani. All Atreides back to the ancient Greek House of Atreus reside in Leto's memory. Leto is a sandworm/human symbiote, having taken the fateful step that his father could not take, and has been tyrannically organizing and homogenizing humans to conform to a "Golden Path" that alone can prevent human extinction. Like Paul, Leto uses prescience to peer through the "veils of time."