I think I need to be more blunt. I would cost shadow+ at 35 minimum (and never use the magic ability). Against some armies, it's can't be killed at all, so it can destroy formation and take tons of pieces for free. Yes, there are some pieces that will be able to push it away or something, but physical-immune is WAY too strong to be on a unit with any unpenalized even mildly offensive moves. The death condition is very avoidable (especially with that movement) and does not change the fact that it can just eat into your army