Like:
- Mechanic of how this keys off getting your opponent below a certain level of Hp, which fits well in Darkness.
Dislike:
- Darkness already has Steal to get through shields, this type of ability might fit better in a weaker Element.
- While they are above 30 Hp this does nothing, then it's an IWIN card (like you'd knock them down to 30Hp and play this for the win).
- Turning off all perm effects with one card is OP. The upgraded attack twice on top of that is way OP, for one card.
- Same problem I have with Nightmare, it doesn't stack. I would like it better if it had some functionality for having multiples in your deck.
Possible alternate version:
Healing effects heal for 1 less HP. (stackable)

: If Opponent HP <30 devour up to 20 quantums. (activated quanta accelerating ability when they get low)