eyeless - It is only listed as a "mythic" because it was printed in From the Vault: Realms. Which is a special collectors set thing. Its original printing was as uncommon.
The ability thing is fine, I will give you that, but I'm sorry but how is Dryad Arbor "NOT an uncommon"? The first time it was printed was in Future Sight as an uncommon.
Ok, I'm stuck. I can't find this last possibility Jones is talkin of. My searches aren't bringing up anything that fits all the clues, given what has already been guessed.
The archive is right, the player can cast the last spell in their hand.
Oppression is a trigger on when you cast a spell. So only triggers after you have already cast your spell, the fact that you have no card to discard doesn't affect the fact that the spell has already been cast.
Blue Elemental Blast
Counterspell
Karoo
Is it a creature?
Basking Rootwalla
Gaea's Herald
Oppression is a trigger on when you cast a spell. So only triggers after you have already cast your spell, the fact that you have no card to discard doesn't affect the fact that the spell has already been cast.
Force of Savagery
Splinterfright
Daybreaker Ranger
Abuna Acolyte
Nefarox, Overlord of Grixis
Belltoll Dragon