Avernus

Avernus was the first layer of the Nine Hells of Baator. The most likely beachhead for any attack by demon-kind, it was the primary battleground of the Blood War legions of devils marched across its plains in continual readiness to repel the hordes of demon invaders that sailed the River Styx into the layer.

Avernus appears in the game in Balders Gate 3 while riding a Nautiloid ship.