Skithiryx, the Shadow Blight

Flying, Haste, Infect, Shadow (This creature can
block or be blocked only by creatures with shadow.)
Whenever Skithiryx, the Shadow Blight attacks,
target creature without shadow gains shadow. If
you control that creature, put a +1/+1 counter
on it. Otherwise, put a -1/-1 counter on it.
Skithiryx, the Shadow Blight gets +1/+0 for
each creature on the battlefield with shadow.