Formatting:
Element should say "Darkness"
Cost should say "2
"
Thread title should have spaces on both sides of the "|", so that it says "Oblivion | Corruption"
You should change the card image for the unupgraded to say "everytime opponent uses a spell" and add "damage is cumulative" in the Text for the unupgraded version so that the table matches the card images exactly.
Mechanics:
May be overpowered, mostly because the opponent has no way to counter damage (without just not playing spells, of course), maybe make it a permanent with an ability that toggles between spell and abilities? This way, it'll be destroy-able (if the opponent is willing to take damage to destroy it).
The introduction of this card would shift the meta-game a lot, since players would be encouraged to have both useful abilities and useful spells in order to counter this one card. Maybe that's too large an impact for a single card?