The Keeper Of Souls is the last boss you will face in Hardmode. It is summoned in The Underworld with a Shaman Voodoo Doll.
When the Keeper Of Souls is summoned it will fire black fireballs (Dark Magic) at the player and dash (Soul Dash) at the player. It will also shoot souls (Soul Blast) at the player. It stays this way until 100,000 HP
Once the Keeper Of Souls is at 100,000 HP a message in the chat appears saying "YOU WILL BE PUNISHED!". In this stage all of it's stats will be doubled.