Complex sentence: Poseidon was venerated at Pylos and Thebes in pre-Olympian Bronze Age Greece as a chief deity, but he was integrated into the Olympian gods as the brother of Zeus and Hades.
Split into simpler sentences: Poseidon was venerated as a chief deity at Pylos and Thebes in pre-Olympian Bronze Age Greece. He was integrated into the Olympian gods as the brother of Zeus and Hades.