Myriam Jahzia, the Mystic, is a crafter that players could unlock in the Reaper of Souls expansion. The purpose of the Mystic is to enable players to transmogrify their items.
The other, perhaps more useful, purpose of the Mystic is to let players change an affix on their weapon, armor, or jewelry (and hopefully end up with a better affix than they started with.)
When I play a Season, I almost always do it in Adventure mode. I try to level the Mystic as quickly as I can. Doing so causes me to miss out on the beautiful artwork that Blizzard created for every level of the Mystic. There are a lot of details to see!
The art for the Mystic’s Level 11 and Level 12 look the same as Level 10.