Hawk, Master of Sins

Indestructible, Vigilance, Hawk must be blocked.
When Hawk deals combat damage while attacking,
you may pay {x}, {x} being the defending creature's
converted mana value. If you do, gain control of
defending creature and attach it to Hawk. Hawk
gains all activated abilities of defending creature,
and may spend mana as if it were any color. On
your next upkeep, the attached creature is returned
to the defending player's control.