Doctor Yersinia Pestis

Wither (This deals damage to creatures in the form
of -1/-1 counters.)
{t}:Destroy target creature with a -1/-1 counter
on it.
At the beginning of each upkeep, if no spells
were cast last turn, transform Doctor Yersinia
Pestis.
The doctor always kept her skin hidden, but no one
seemed to notice