Final answer:
Agamemnon is the former king of Mycenae, brother of Menelaus, and the commander of the Achaean forces encountered by Odysseus in Hades from Greek mythology and the Trojan War.
Step-by-step explanation:
The former king of Mycenae, brother of Menelaus, and the commander of the Achaean forces in Troy whom Odysseus encounters in Hades is Agamemnon. This character is part of the ancient Greek epic, the Odyssey, and his story is intertwined with other key figures from Greek mythology and the Trojan War. Agamemnon's narrative is pivotal to understanding the tragic cycle that affects many heroes including Achilles and Ajax, as it encapsulates themes of honor, betrayal, and revenge in classical literature.