You may want to contact the Game Developers to see if there is a solution. The material that Yagluth drops is needed in the Mistlands to help you collect materials so you can explore the Mistlands and for at least one new weapon (Mistwalker) that I noticed.

Without it, it is conceivably possible to do the Mistlands, but it would be extremely difficult.

The run I am on now is from a world I started in early fall, then stopped playing before killing Yagluth. When Mistlands was fully released, I resumed playing it, killed Yagluth, and got the new materials. So it is possible to get the materials from a pre-Mistlands release generated-world.

I would contact the devs for advice. There may be a way they can restore the items or a previous save.

Best of luck.