The Chest of Yuul is a chest that can be opened with a Key of Yuul obtained from a Wormfeeder Rune. It and all other Wormfeeder Rune chests will reward engrams, Court of Oryx runes, Wormspore, and Calcified Fragment XXXVIII: The partition of death.
The chest is located in the Hall of Souls on a high ledge on the opposite side of the entrance to the Hull Breach. Two small transparent platforms are available to jump on to make it across.
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Yuul Chest Location Guide - Destiny-Yuul Chest