Caustic, lethal breath

Defender
When caustic enters the field, he gains X toxic counters where X is the number of black creatures in your graveyard.
Necessary evil: {g}{b} send a green land you control to the graveyard. If you do add a toxic counter to caustic.
Poison mine: {b}{b}{g}{g} at the beginning of your upkeep spend a toxic counter to give a target creature deathtouch until the end of turn