How To Immobilize the Priest

Pulverize the enemy Priest with your battle units. When you've reduced him to half of his Hit Points, he'll stop moving, drop anything he's carrying, and a thin force field will form around him, keeping him alive.

A Priest will not be able to cast a Spell while immobilized nor will he be able to construct buildings.

Notice that the Priest is constantly regenerating Hit Points, so be sure to keep up a steady barrage or he may regenerate himself enough to flee.

Next: Use a Transport to pick him up and carry him to your Altar.