How do I get demonology hidden artifact?
Can you still get Mage Tower artifact Skins? The Mage Tower Challenge is being removed in the Battle for Azeroth pre-patch. If you want to use this appearance for transmog, you must complete it in Legion!
Can you still get ghost of the Pridemother? Ghost of the Pridemother: Unlocked through the Mage Tower Artifact Challenge, An Impossible Foe. Note: The basic tint of the challenge appearance is unobtainable as of Battle for Azeroth prepatch. The other tints can still be unlocked if you finished the Mage Tower Artifact Challenge during Legion.
Can you still unlock Legion artifact Skins? Yes you can. All artifiact appearances are still available except for Mage Tower challenges. The latter ones were meant to be a solo challenge to unlock a cosmetic and would be trivialized by level 120 characters.