Rav, Double Edged Lightning

Vigilance, double strike
Whenever Rav, Double Edged Lightning attacks,
target attacking creature gets +2/+1 and gains
double strike until end of turn.
Sacrifice a creature: Put +1/+1 counters equal to
the sacrificed creature's power on Rav, Double
Edged Lightning. Rav has all abilities of creatures
sacrificed with it.