Hey, I like this! I've got some suggestions as an avid card game player, hope you don't mind. Feel free to ignore them though, won't hurt my feelings!
First thing I see is, I don't like the idea of requiring one specific card (hardmode) to enable most of certain decks. The options I would consider are: Add an easier way to draw into hardmode specifically, or a new condition. It doesn't follow the game perfectly, but maybe instead of having hardmode be a card, just make it something that happens when a boss monster dies? Alternatively, it could still be a card that goes into decks, but when a boss monster dies, you can search for it and add it to your hand? Also, maybe make it affect each player, so one doesn't get stuck in easy mode and completely demolished by the superior hardmode cards. Of course, this would make the opponent's hardmode a completely dead draw, so it isn't a perfect fix.
Secondly, and this is just me personally, I love tribal decks in MTG so I'd like to see lots of synergy between certain cards; more definitive archetypes. I like the slime theme you have going on, and I think the biome borders are a great idea! I know they'll be expanded on in the future, so I'll reserve judgement on that for now.
I'd also like to suggest some flavour text! Some wording on the cards could be improved as well, here's how I would word some:
Green slime
This unit has +1 attack point for each other friendly Slime unit.
Flavour text was good on this one!
Blue Slime
As long as you have 2 or more friendly Blue Slime units, this unit has +2 defence points.
This one is fine as well.
Side-note: I'm not sure if this would be too strong, but I would like the blue slime to require any slime to be stronger, not specifically another blue.
Pinky
Looked good to me, but the effect doesn't seem terribly fitting for a slime. I find it a huge pain to deal with unless it's alone, so maybe make it stronger for each slime you have active? I'm totally trying to push a theme here...
No changes here.
Zombie
Friendly Demon Eye units have +1 Defence point.
Blue Slimes are not the worst of your problems....
Side-note: I couldn't think of a good way to word it to mean exactly what you were going for, so I figured the easiest way was to spread the ability across both Zombies and Demon Eyes, as it needed both to function anyways. The current wording is a bit confusing, I'm not sure what would happen if you have multiple zombies. Depending how many zombies/demon eyes can be on board, this might get too out of control, so maybe a clause would have to be added that even If you have 2 zombies, the demon eyes would only have +1 defence.
Demon Eye
Friendly Zombie units have +1 Defence point.
If this unit receives a fatal blow, you can destroy a friendly Demon Eye unit instead.
The bloodshot eyes of a moonlit night.
Ice Slime
No changes here
When this pouncing blob shivers, it isn't from the cold.
Ice Bat
When this unit deals a fatal blow to an enemy unit, that unit's controller cannot use any new research cards during their next turn.
YOU try innovating with cold fingers!
Side-note: I worded this one under the assumption that 3+ player games are possible. If this isn't the case, I would replace 'that unit's controller' with 'your opponent'.
Wolf
Whenever an opponent activates a Condition Card while you are attacking, it will not take effect until your attack is done.
If you're cool with saying '', then I have no problem with it
Corrupt Slime
When this unit receives a fatal blow, look at one card in an opponent's hand, chosen at random.
This flavour text is pretty fitting, I can't think of anything better
Side-note: Again with the multiplayer thing, if it's only gonna be 1v1, then replace 'an opponent's' with 'your opponent's'.
Eater of Souls
Whenever an enemy unit attacks this unit, it loses 1 attack point for the duration of the attack.
From the corrupted chasms rises a new apex predator...
Side-note: I am unclear as to whether this attack point drop is permanent or not, so I worded it as if it only applies for the one attack. If it's permanent, I would simply say 'it loses 1 attack point'.
Devourer
Whenever this unit inflicts a fatal blow to an enemy unit, that unit's controller discards a card from their hand.
Nothing can stand between a Devourer and it's prey.
Side-note: In the case of more than 2 players, change 'that unit's controller' to 'your opponent'.
Pixie
Whenever you inflict a fatal blow to an enemy unit, draw a card.
With the spirits of light released, a new plague seeks to corrupt the world. Not through blood or shadows, but with the power of rainbows.
Unicorn
Whenever this unit attacks an enemy Corruption unit, it is immediately destroyed.
Cute but deadly, like a charging hippo.
Side-note: An alternate wording could be 'Any attack directed at an enemy Corruption unit by this unit is considered a fatal blow'.
Antlion
Whenever this unit attacks an enemy unit, your opponent cannot activate Conditional cards for the duration of the attack'.
Many underestimate the Antlion for it's lack of mobility. Many lose their lives to a long-range blast of sand.
Side-note: Same deal here, it may need to be worded 'your opponents' or 'that unit's controller', whichever is intended.
Antlion Charger
If you control another Desert unit, this unit cannot be attacked.
A distracted foe will never keep up with an Antlion Charger's incredible speed.
Snatcher
Whenever this unit inflicts a fatal blow to an enemy unit, that unit's controller loses 1 Hearth Piece.
These plants have adapted a long stalk to reach into unsuspecting adventurer's tunnels and snap them up.
I could do more if you like, but that's probably plenty for now. Apologies for being such a nit-picker, but consistent wording is pretty important to keep rules clear!