When the Black Fleet arrived, the Cryptarchy discovered that Umbral Engrams and Legendary Engrams are generated through similar processes. How or why the engrams change is still a mystery.
However, we now have a tool for controlling the contents of these Umbral Engrams! We call them Prismatic Lenses. The Drifter has yet to divulge how they're constructed, and truth told, it's possible he doesn't know either.
Here's the final Prismatic Lens remaining from our previous studies. It's quite fragile, and we estimate it has a single charge left before it breaks. Take it, along with this Umbral Engram, and try the focusing process for yourself!