Sorry, I've been away for awhile. You can actually play pretty decently with less mana if you build your decks smartly. You'll probably be relying a bit more on your first draw for lands though. In general, if you can get 4 to 5 lands in your first 5 or so turns, including your original draw, you should be doing fine. While I have a lot more spells than lands, most of the spells cost 2 or 3 mana, The real trouble for this deck is that I made it when I had a lot fewer cards, so it just isn't as effective as a blue/black deck should be.
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The other colours part ain't a big deal thanks to prophetic prisms. I do okay for mana in play, but I probably will get rid of the discard part (memory sluice discards, but I'm about to trim down this deck anyway).
Door of destinies is great but thrumming stone has a chance of killing overrun or overwhelming stampede. If I have a rat army it should have trample.
4 coat of arms = +5/+5 per rat.