For the Hephaestus Forge rituals in Valhelsia for, I cannot find any means of creating or obtaining a darkstone or arcane darkstone pedestal. The JEI shows no recipe and the wiki doesn't explain how to craft it. I've googled all over and found no instruction videos. Is this a multiblock structure? Perhaps this is a Valhelsia-specific issue where they removed the recipe?

Edit: nevermind - looks like there's no recipe for this and it's likely a Valhelsia 4 issue.