Make Them Pay seems interesting... I feel like it should be damage-based though. Unless you want someone to play Plunge into Darkness
or Toxic Deluge
and then hit an opponent for 19 (or more potentially). Then again, that's a turn 6 thing, so... I guess it's fine?
Imperium is probably fine; in 1v1 it's basically a cheaper Mindslaver
with color requirements. In multiplayer it's pretty strong, but, it's an 8-drop, so, probably fine.
Desperate Pact is a bit weak, I think. Compare Eldrazi Monument
(similar effect, but stronger, and only 1 sac per turn) and Phyrexian Soulgorger
. Cumulative upkeep - Sacrifice a creature is a strong enough drawback to get a colorless 8/8 for 3. I guess the Platinum Angel effect is also useful, but not as much as it could be... honestly? I could see making it cost
, or even
, and still adding Flash to it. The flavor is really cool though, and gives me some interesting ideas...