Diosdado is the watchman of the Library of the Negated Words, carrying a ring of many keys and even more keys around his neck, as well as a lantern. These keys supposedly unlock the shelves of forbidden volumes kept in the Library.
Found in the Library of the Negated Words, Diosdado begs the Penitent One to stop the sound coming from below. Stopping the sound requires using the Shroud of Dreamt Sins to reveal a ghost who is banging on a wall deeper in the library. After taking down the ghost and the breakable wall it was banging on, and entering the new room and exiting, Diosdado will appear. After talking to him, he rewards the Penitent One with The Young Masons Wheel.