The Island of the Sunless Sea is a Nephalem Rift accessed by going through the red portal created by the super unique GrubberGheed's Mentor when he dies. He's found in the Treasure Vault, the second level of the Stony Tomb in Act II.
There's a 10% chance that those bosses drop the statue Sunless Crystal Bird. Its only purpose is to add a small bonus to the charm when transmuted with it.
The place where angels watch, an underground cave miraculously illuminated by sunlight. Was once the staging ground of the heavenly hosts for their crusades into the abyss. When the holy legions were defeated and forced out of the dimension by the forces of Hell, a few human worshippers remained behind to stand vigil. However, heavenly magic is too powerful for mortals to tolerate for long. The mutated, wretched remains of the human defenders were found and embraced by the mighty Lucion, son of Mephisto.