It's a good idea,
but when the opponent had Oty's or even Lightning,
This card, would be wasted,
I think the main threat is Oty's and a Scarab Army.
You could say, that it enters the game immortal, but the fifth turn it becomes mortal again,
because otherwise, It would just be eaten/lightninged etc. So I don't know, just an idea?
Overall, good card!