Mistical Shadow Mimic

Flying, deathtouch, lifelink
All creatures you control with {b} in their mana
cost get +1/-1 or -1/+1, as long as this creature is
on the battlefield. If a +1/+1 counter was dealt
this turn, instead they get +2/-1 or -1/+2
instead.
All creatures with power 6 or greater cost {5} less
to cost as long as you've played a creature,
instant, and sorcery spell this turn.