Dark mark is definitely the right choice. I tried something like this without a dark mark once and you can actually deck out if you bottomdeck a bunch of the void shards. I think I'd take out one deadly poison and add one of some other cheap kill card, the reason being that you will autolose against a deck that plays a few early purifies otherwise. If you went with 1 deadly poison 1 dirk, for instance, you'd have two different ways of dealing damage, and so they'd need to have more counters to block both. (like purify and dimshield, for instance) Siphon life could also work, as could a nightmare, though that last is risky due to requiring a creature target, which you have none of. I picked dirk over anything else because it gets around being locked down by amber nymph or pestal or something like that.
