Half-serious suggestion: Change the Upgraded card to Soul Mate with the Deja Vu ability.
XD That's an amazing idea, if not completely cheesy and hilarious.
This card seems to be balanced ONLY because of the fact that there are a lot of things you have to consider. Unupped, it's nice to have an immortalized creature with Eclipse activated (dealing 2 damage a turn). Upgraded, even with its ability to consume things to get stronger, I would suspect that with an Eclipse out (to eat X|1 critters), skeletons are automatically removed from this equation, since they're also buffed by Eclipse/Nightfall.
Which still leaves Sparks or Photons as the best options for Lost Soul fodder. A smart idea to keep this immortal/untargetable. Decay is a pain an annoyance to fight against, and having Fractal on these guys would make me want to gag every time I faced a Mono-:darkness with these guys.
The art is cute, but it seems
slightly... rushed?
Great card nonetheless.