Dawn Chorus

Rahool is the answer. Dawn Chorus is in the world Exotic pool.

His decoding menu focuses a named Exotic armor piece for an Exotic engram and an Exotic Cipher.

Failing that, Exotic engrams run on a knockout system: anything you don’t own is likelier than anything you do. Opening them blind gets there reasonably fast on a Warlock without much of a helmet collection.

Legend and Master Lost Sectors drop Exotic engrams, so soloing them feeds Rahool rather than targeting the helmet.