Gem of Poisons

When this artifact enters the battlefield, put a -1/-1 counter on each creature you control. Those creatures cannot be blocked until end of turn.
Outbreak {3} ({3}: Creatures with a -1/-1 counter on it gain infect until end of turn.)
Whenever a creature with a -1/-1 counter attacks, you may pay {1}. If you do, that creature cannot be blocked until end of turn.