You'll find the Rot Knight at the bottom of an area called the Sarcophagus Rift, which you can access by descending through the rift. However, before you can fight this boss, you need to have completed the main story first, at least as far as Shadow Keep.When you reach the keep, a message will pop up saying "Somewhere, a great rune has been broken... and a powerful amulet." You can't reach the...