In Dark Souls, which boss is fought in the painted world of Ariamis?

In Dark Souls, Crossbreed Priscilla is the boss fought in the Painted World of Ariamis.

The Painted World is an optional area accessed through the enormous painting in Anor Londo. It contains hostile inhabitants, a snow-covered landscape, and a route that eventually leads to Priscilla. Unlike many required bosses, she does not have to be defeated to leave the area; the player can pass through the arena and jump from the far edge to return to Anor Londo.

Priscilla is notable for her invisibility mechanic. After the battle begins, she can disappear into the snow, leaving footprints and other visual clues that reveal her position. The encounter also offers a special choice: attacking her tail can produce the Lifehunt Scythe, while leaving her alone allows the player to exit peacefully.