Ouch. This is pretty overpowerful. If you look at other immortal cards in the game:
Immortal costs 6 and does 4 damage.
Upgraded, it costs 7 and does 5 damage.
Phase Dragon costs 13 and does 8 damage.
Upgraded, it costs 14 and does 10 damage.
As you can see, across the board, the costs outweigh the damage. The unupgraded cost/damage ratio is an average of 1.5625 and the upgraded ratio is an average of 1.4.
Your creature has a cost/damage ratio of .875. This is not only insanely good for an immortal creature, but it is good for a regular creature that does this much damage, also. Very cheap and immortal, and it does 8 damage?!
Not to mention its ability, which allows it to grow even MORE powerful. All in all, it's quite a nasty card.
Plus, I like to think of Aether cards as the only ones that have Immaterial cards. It's one of the things that makes Aether special.