Kasumi, Misty Reflection

Whenever one or more creatures attack, put a
Mist counter on target attacking creature's
controller.
{1}{g/u}{g/u},{t}: If a source would deal damage to a
permanent or player this turn, if that source's controller has a Mist counter on them, it deals
that much damage minus X, where X is the
number of Mist counters on the source's
controller.