I got a Spark when I defeated Lilith and it won’t let me use it to forge an Uber. I have it in my inventory but when I go to the blacksmith he tells me that I am missing materials.
For a random Mythic you need 2 sparks and 50m gold.
I’ll note that the new materials breakdown UI here is somewhat “secret” – the materials icons and gold are shown, but you need to invoke the “Show Materials” option on that card (e.g. square/X on controller) to actually have the game reveal the full breakdown of required materials.