The Cave Ordeal

Dumbledore and Harry Apparate to a sea cave. They enter through a hidden passage and find a vast underground lake with a green light in the center. Dumbledore summons a boat, and they cross to a small island. There, they find a basin filled with a potion. Dumbledore determines it must be drunk to reach the Horcrux at the bottom. He drinks the potion, which causes him immense pain and suffering, forcing Harry to make him continue. After Dumbledore drinks it all, they retrieve a locket from the basin. As they leave, Inferi attack, but Dumbledore uses fire to repel them. They escape the cave, but Dumbledore is severely weakened.