Found by Fernweh after the Battle of the Hall of Misjudgment, 3-35, after he fell into the eastern of the two pits.

Ring, legendary (requires attunement)

While wearing this ring, you can turn invisible as an action. Anything you are wearing or carrying is invisible with you. You remain invisible until the ring is removed, until you attack or cast a spell, or until you use a bonus action to become visible again.
— Basic Rules (2014), pg. 191; see also D&D Beyond