Power Stone

Whenever a creature you control attacks alone, it
gets a +1/+1 counter and gains trample until end
of turn.
{1}: Until end of turn, whenever a creature you
control gains a +1/+1 counter, it gains an
additional +1/+1 counter. Activate this ability
only once each turn.
"The six stones were scattered across the universe;
only to be brought together for destruction."