Hiroko, Snake Paladin

Whenever a Snake enters the battlefield under your control you may put a serpentes counter on that creature. Snakes with serpentes counters on them have Deathtouch and "Whenever an opponent's creature dies, you may remove a serpentes counter, and if you do, put a +1/+1 counter on this card."
Sacrifice 5 creatures with serpentes counters on them: Each opponent gets a poison counter.