Hellcastle

The Hellcastle is a structure that generates on world creation. The Library of Knowledge lies at the end of the Hellcastle, but is only accessible in hardmode.

Library of Knowledge
In order to enter the Library of Knowledge, you must craft 2 Impervious Keys using 2 Temple Keys, 10 Ectoplasm, and 1 Earth Stone at a hardmode ore anvil, and unlock both doors at the entrance. Once inside the Library of Knowledge, there is a crafting station used to craft the Ectoplasmic Beacon to summon Phantasm. There is a Devil's Scythe tome on the shelves, as well as many books.

There is also an Underworld Chest on the shelves on the second room. This chest always contains the Boomlash, as well as a chance at the Vampire Teeth and Gold Coins, the Vortex accessory, potions, Impervious Bricks, and Ectoplasm.