Exile pacts with you Nivmagus, so you don't die next turn!
I built this deck, because I originally thought about making an infinitely strong Nivmagus, by copying a copy spell repeatedly. While I was researching to see if it was possible (which it isn't), I remembered pacts. They costed 0 mana, so I could play as many of them as I wanted. And then if I exiled them, they wouldn't resolve, and I wouldn't have to pay the mana. So now, the Nivmagus is strong, but I won't be able to swing for the win. I thought about attacking for 10, then flinging for the last 10 damage, but that wouldn't be very reliable. Double strike or infect would work, though. And I could use Apostle's Blessing to make it unblockable. It's perfect. Now I can win second turn, and if I don't... well if I don't, let's just hope I won't be dead by third turn. :)
I added Kiln Fiend and Flamekin Harbinger in case I don't get the Nivmagus. Thoughtseize and Apostle's Blessing are for defense. Assault Strobe and Tainted Strike allow for a second turn win. Obviously, the pacts are for exiling. Gut Shot is for when the opponent has a 1/1 chump blocker that needs to be disposed of, and Noxious Revival is for when Nivmagus is dead, and I don't have any other creatures. Both of those can still be exiled.
I'm limiting the deck to modern, but there is no budget. However, it would be nice if it was a bit cheaper. I would like the deck to remain Rakdos colors, but if there are really good non-Rakdos cards, I'm open to suggestions.
I have not yet playtested, as I'm way to cheap to buy this deck, so I'm not sure how it plays. I'm guessing that 17 lands is enough, because most of the spells in this deck cost nothing to play, and the only card I'll be playing for 2 mana is the Kiln Fiend. Try to hold back on playing all your spells until you know you can win, because unless you have an Apostle's blessing, Nivmagus dies really easily. If you have a Nivmagus in your hand, don't bother with the Flamekin Harbinger, unless your Nivmagus dies. The perfect hand/cards to draw contains a Nivmagus, 4 pacts, Tainted Strike, Apostle's Blessing, and 2 lands.
Game plan: Play the Nivmagus. Dump all the pacts, and exile them all for hopefully a 9/10 or 11/12 Nivmagus. Use Assault Strobe if he's 11/12, or Tainted Strike if he's 9/10. Use Gut Shot or Apostle's Blessing to eliminate any chance your opponents might have at blocking. Keep in mind that you can let the some of the pacts resolve if you need to, if you know you'll win before the next turn.