Ok, a lot of good cards here
Assasin kills a targeted creature with less attack? In my opinion that's stronger than otygh, it grows faster. Still the cost is too high, what about 6
?
Upgrader has been suggested before, I don't remember who posting it. It was called evolution or something, the upgraded would turn the creature into it's unupgraded form
Rebirth, not possible, and way too cheap, bring back a dragon for 2
?
There is no graveyard in the game
You could maybe change it to make the first creature supposed to be killed get back to normal stats or something (if that made sense) but why not just quint it.
I think field freeze is a bit OP, it's just a feeling I've got, putting out all creatures for 3 or 4 turns?
Lava is fine, nightfall effects both death and darkness creatures, think about that
that's it!