Achilles, Shining Wrath

Achilles, Shining Wrath's power is equal to
the number of creatures target opponent
controls.
You may exert Achilles, Shining Wrath when it attacks. When you do, it gains Indestructible and Double Strike. (An exerted creature won't untap
during your next untap step.)
"Lion-hearted Achilles, cleaver of the ranks of men..."
--Ajax the Great