I just wonder whether the skull is in the drop list.
My Forge with 169 Luck over 100 waves and carry with 1753% item chance did not get Skull neither.
Wouldn't it be hilarious if the skull item was just a placebo? "LOOK AT THIS AWESOME THING YOU UNLOCKED! YOU'LL NEVER SEE IT."
I fear this is indeed the case. In the current version, the dark forge is not able to drop legendary items (which is because the forge tower from a time where no legendaries did exist). Now, the skull is, in fact, a legendary and will never drop.
Sorry I didn't catch this for this release! Always tested with the skull added to the inventory, never thought about there might be an issue with the forge itself.
Trying to release a hotfix for this (plus a few other things reported for this version) as soon as possible.