What is the source of the Mages’ power?

I've never played as mages on large maps, but they really shine in the late game. I saw a video from a popular blogger about the fifth game, and he built a mage with almost 50 knowledge. As a result, his high-level creatures had +11 to attack and defense, as well as about +50% to initiative. Considering that the mage's army has 3 archers, this initiative bonus is a huge advantage, as it allows him to destroy enemy units before they even get their turn.