From Destiny 2 Wiki
|Description||A device to extract the Darkness from Umbral Engrams and reveal the equipment within. Built by the Cryptarchy, this version is safer than the Drifter's original design.|
Umbral Decoder is located in the Annex part of The Tower, right in the Drifters room. To gain access to it you'll have to complete Invasion Site event on Io or Titan, which is a part of In the Face of Darkness quest.