In order to do this, you must have the Frostcrag Spire expansion. Im not exactly sure if it works if you buy it on marketplace because i got it from a magazine disc. But anyway, you must finish the quest to restore it. Once you restore it, go to your bedroom, and go onto the thing with all the plants. There is a rock there. Stand in the corner of the rock. save it right there, and then delete the expansion.Then, go to that save, and the screen will be black. immediately walk forward, and eventually, youll be in a house. go downstaris, outside, and the mace of doom will be somewhere on the right. There is an oblivion gate type thing, but i forgot where it was, so just walk out of that. Then, save it, and download it again. You should have frostcrag spire restored and the mace of doom in your inventory. Hope it works!:)