Diffusation|Dissolvation

[Arum]

4. Maybe make this target ANY card? Would that work?

Expanding the range past creatures would actually make this worse balance-wise.  Maybe try something different than straight up destroying the creature.  Maybe dealing a large amount of damage, maybe 5.  That definitely softens the blow while still being able to kill most things.  From there, it'd be easier to balance the costs to Firestorm.

And thinking about it more, the name does not fit the effect and the unupgraded name doesn't really fit the element.

Okay, DRASTICALLY changed the cards. Better now?

Yes,  But with the change in effect, does it really fit into ?

Yes, because its taking the HP away (draining it) and giving others the HP (vampiric effect.)

[Arum]

Seems more of a gravity card or something to me - Darkness just doesn't fit thematically. Dissolvation and Diffusation don't make sense either - just use Diffusion|Equilibrium for card names. They're actual words and fit the effect.

I rather prefer making up words, it's just something I do.
It fits darkness because it's diffusing/dissolving the hp (life drain) and healing others (vampiric effect).