Invasion of The Dross Pits

(As a Siege enters, choose an opponent to protect it.
You and others can attack it. When it's defeated, exile it, then cast it transformed.)
When Invasion of the Dross Pits enters the battlefield, put a -1/-1 counter on each creature,
then proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)