hmmm... sounds more like an entropy weapon than darkness, don't you think?
3 multi-cast would be fine.This is not very effective against rainbows 'cuz rainbows use QP(/QT, same below). You need 9 each turn to nullify the quanta produced by 1 QP. And rainbow typically have >10QP, so you need 90 /turn to counter it, which is impossible.
This should be dark. I don't see some connection between this and entropy, and dark is about sneeky things. This card can "sneekily" (sort of) dry up your quanta pool
The cost is still low. Mono dark can build quanta quickly enough to fully deny the opponent. Make it 3 and destroy 1 random for each 10 you own. 1 turn 1 cast of course