Interesting idea. I've been contemplating something similar for permanents, but it seems tricky to make without breaking the game.
To me, this card just seems like it will be too hard to balance right as it is. I think adding a limited duration here would probably help a lot though.
From a defensive point of view, this effect is like an ultra effective creature only cloak. Since cloak has a limited shelf life, i would expect this should to.
Offensively, it does give a means to counter immortals directly without needing shields. But again, this should probably be a temporary thing or else it kind of defeats the point of having immortality to begin with.
One very fun feature I can see is that it would allow you a chance to heal and buff your own immortal creatures which wouldn't be possible before. This would make for some fun aether duo decks.
Limited duration also fits the "Idun" theme since she did get returned eventually.