Fortiam, the Unmatched

Trample
Whenever Fortiam, the Unmatched attacks, you may pay {1}{g}{g}. If you do, Fortiam gets +X/+X until end of turn, where X is the greatest power among creatures defending player controls.
The last remaining buffalo of this land, Fortiam, lives with the humans who have constructed his armor and fixed his broken body parts. Because of that, the ancient buffalo protects his only friends.