Ple-Sen
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Lure him into your castle (there should be a hero stationed in the garrison). Make sure he stays there until your turn, then enter the castle and press the spacebar. After that, the hero will seemingly "consolidate," and you'll be able to control him. However, it's likely that his AI will glitch, causing him to wander back and forth during the ally's turn, and during your turn, he'll just sit there with unused movement points.