Mordred

If Mordred hasn't been exerted this turn, you
may exert it as it attacks. When you do, untap all
other creatures you control and after this phase,
there is an additional combat phase. (An exerted creature won't untap during your next untap
step.)
"I am no king, but I follow in the king's path. I will destroy all that I must to bring the king peace!"
—Mordred