Decrypting the Darkness
The Umbral Decoder and Prismatic Recaster are located within the H.E.L.M. Visit the Umbral Decoder to obtain the "Decrypting the Darkness" quest, then complete it.
Used correctly, the Umbral Decoder can change the contents of Umbral Engrams, granting potent rewards. Interact with it in the H.E.L.M.