The Chest of Eir is a chest that can be opened with a Key of Eir 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 Hull Breach in a chamber on the far right side near the entrance to the Mausoleum.
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Eir Chest Location Guide - Destiny-Eir Chest