After the Witches festival ends, the cauldron will not turn you into a robed skeleton or grant an XP bonus until the next witches festival in 2019.
Notably, (not in relation to the witches festival) if you completed the quest in stillrise village, you will get a skeleton polymorph, and there is also the "Cadaverous Assassin" polymorph which is a skeleton polymorph garbed in the ways of the dark brotherhood which is acquired by completing the "Litany of Blood" after reaching a certain rank in the Dark Brotherhood Skill line.
Edited by Skullstachio on October 19, 2018 12:56AM
If you see me anywhere. Know that I am sitting back with a bag of popcorn, watching as ESO burns the goodwill of its player base with practices that only disrespects the players time like it did to me and many others...
If a game does not respect your time, best thing to do is move on from it and find something else.