Plague Monk

Plague-Addled (all colorless mana must be paid by sacrificing a land or paying 3 life)
Rot (creatures damaged by the Plague Monk gain a decay counter, and they gain Decayed after they reach 3 decay counters)

{c}Pustulant Mixture: Only usable during beginning step, gain Double Strike until beginning of end step. If no creature was defeated by the plague monk when it ends, send it to the graveyard.