Alcyoneus

Alcyoneus is one of the giants created by Geae to destroy the gods. He is the giant created to rival Pluto.

History
During the first Gigantomachy, Alcyoneas couldn't be killed as long as he was within his home territory, Pallene. During the war, Hercules fought him in battle and wounded him, but couldn't kill him until he dragged him out of Pallene using his great strength

Before The Son of Neptune
Back during World War Two, Gaea manipulated Hazel's mom, Marie Levesque, into moving to Alaska, an area out of reach of the Olympian gods. Geae would take control of Hazel's mother ever night and forced Hazel to call up precious stones and metals on a phantom island off the coast. The almost lead to the raising of Alcyoneus until Hazel sacrificed herself to put the giant back in the earth and sink the island.

The Son of Neptune
He reformed several decades later and attacked a Roman legion that had gone up north to try and fulfill the prophecy of seven, but ultimately failed and lost the camps sacred symbol, an eagle staff.

Hazel, Percy, and Frank travel north to Alaska and face him in combat where is is eventually defeated when Arion, Hazel's horse, drags him into Canada and he is killed by a combined effort from Frank and Hazel.