While this would hurt the Rainbow in PvP, it would also be a huge boon to anti-FG rainbows. Darkness is underused in them (a steal or two maybe, I've seen arguments that the steal is not worth the space it takes up in the deck posted here), and having two of these would really limit the advantage drawing two cards a turn gets you.
Also, imagine this in a Quantum denial deck (pests+this+earthquakes+whatever creatures you want). Destroy their pillars so their hand gets clogged with creatures that they have to discard. Eliminate as many as possible of the quantum they do gain with pests.
Something like
Mark of Earth
10x Obsidian Tower
5x Earthquake
6x Pest
5x Vampire
3x Oblivion
1x Nightfall
Or something along those lines.
I like it.
EDIT: I realize the deck might benefit from a few drain life either for direct damage or creature control of anything the opponent actually manages to get out. I'm not sure how to build really good decks, so I don't know how to fix the balancing, but the idea is there.
