How to promote Monster Classes, and how to change promotion item for classes?

FE8. I just got Gilliam, my Revenant, to lvl 11, and I tried to promote, but it won’t let me. He has a Master Seal, and I have it set in FEBuilder that Revenants can promote to Entombed… Why won’t it work? Do I need a specific item, rather than a Master Seal?

Did you set the Master Seal to be able to promote Revenants in the Promotion Item Editor?

I absolutely did not know that was a thing. No, I’ll go set those now, lol. Thank you very much.