I am unable to use the Prismatic Recaster on my Warlock or Titian. It says, “Requires “Decrypting the Darkness” completion.”
The quest is not current in my active quest log, kiosk nor is it being offered by the umbral decoder.
I have it done and completed on my Hunter and am able to use it on that account.
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Try visiting the War Table, not at the main interaction point, but slightly to the left, where a datapad lies.