Cabal Summoning

As an additional cost to cast this spell, tap any
number of creatures you control.
Search your library for a Demon creature card
with power X or less, put it onto the battlefield,
then shuffle your library. Then all other
creatures get -X/-X until end of turn, where X is
the number of creatures tapped this way.
Demons are rarely benevolent to anyone, not
excluding to their summoners.