When the community says please give priest a win condition I don't think this is what they meant. That said, this doesn't help priest as much as people think it does.
While obviously curvestone could occur with a turn 10 kill, aggro has been rampant and not enough has been seen to show it going in a control direction. This card doesn't really help priest win the aggro matchup, it's obviously just in line in a combo matchup, and in theory this would win in a control matchup. However if the quest existed in a control matchup you'd save mutanus for the turn the quest ended to eat xyrella, or in warlocks case Tickitus could just burn the shard itself. While this could be good I don't see it being nearly the gamebreaker people think it is.
Honestly at best a 2 star with current kits, and i'd bet it gets nerfed if it is too strong anyway.