Whatever colors you go with, make sure to have plenty of enchantment removal or permanent bounce just in case the hand-off from Harmless Offering doesn't go as planned. would suck to get stuck and die to your own win condition.
Since you're already in black, you could add the
Tree of Perdition /
Triskaidekaphobia combo to it, and just make it a general black/Red combo deck. be like a weird version of the old
Dark Depths / Thopter deck. Some people will be able to stop the tree and others the Pact, but in general they'll struggle to stop both. Just something to think about.