Mors Aeterna

Whenever one or more -1/-1 counters would
be placed on a creature you control, instead
put that many +1/+1 counters on that creature.
Whenever one or more +1/+1 counters would
be placed on a creature an opponent controls,
instead put that many -1/-1 counters on that
creature.
{w}{b}: Put a -1/-1 or +1/+1 counter
on target creature.