Mad Rider

Menace
Whenever you discard or mill one or more cards, put a +1/+1 counter on Mad Rider.
Madness {1}{b} (If you discard this card,
discard it into exile. When you do, cast it for its
madness cost or put it into your graveyard.)
"Whether or not you are cut down by their horse charge, it's just a matter of madness"
—Rhyta