Phlegethon (Final Fantasy XIV)

Acheron (アケロ－ン, Akeroon) is the boss of the Crystal Tower: Labyrinth of the Ancients and is fought in the upper area of the Labyrinth after the first 3 bosses, Bone Dragon, Thanatos and King Behemoth, are defeated.

Appearance
Acheron appears as a giant, golden humnaoid wielding a large curved blade and black patterns or tattoos across his body. His blueish cape seems to be attached to his red collar. He has yellow hair.

Final Fantasy XIV
After Bahamut's assault on the lands of Eorzea, the Crystal Tower that lay buried in Mor Dhona rose above the surface. Fearing the tower may contain Allagan weapons similar to the Ultima Weapon, several heroes from Eorzea are gathered to venture inside the tower. After overcoming the trials of the elemental Gates, the adventurers break into the Labyrinth around the tower. As they ventured through the dangers of the Labyrinth they come across Acheron, who guards the exit leading to the base of the Tower itself.

Final Fantasy XIV
Acheron is battled in a large circular arena with a gutter circling it. In the gutter sits 3 buttons that for most of the fight are submerged in Lava. His fight serves as one of the first time unique AoE warnings are shown in place of the usual red circle or line. Throughout the fight, Acheron will sometimes begin casting a destructive AoE that strikes the entire arena, and shortly before he casts this spell the Lava covering the buttons is drained. Players need to move into the gutter and stand on all 3 switches to form a barrier that protects them from the damage, otherwise the entire Alliance takes unavoidable 9999 damage, causing a wipe.

Final Fantasy III
Acheron made his debut in Final Fantasy III and appears as a strong enemy throughout the game.

Etymology
is a river in Greece, and according to Ancient Greek mythologies, was one of the five rivers of the Greek underworld along with a rearranged version of Battle 2 theme.