Mesmer, Dream Walker

while casting this spell you may remove 2 dream counters from cards in exile to pay for each {u} or {b}.

Whenever a card an opponent owns is put into exile put a dream counter on it.

{t}: choose one. Pay the cost for the chosen mode
•Remove 3 dream counters from among cards in exile: Exile target creature.
•Remove 5 dream counters from among cards in exile:
Copy one of those cards you may cast the copy.
•Exile the top card of each opponents library.