you don't just control them; they're in your party. if you want to just control them, use Illithid Control Circlets

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Thus, you cannot control more than 5 of them, and that requires booting out all other NPCs. They are then, for all intents and purposes, IN your party. you can edit their spells, command them, equip items on them, etc. They also never turn on you if damaged. So you could go through the game with a horde of vampires, but they would have to be unnamed and unknown. For vampires, though, on second thought, their death might cause the gaseous form transformation, meaning you would lose them and they couldn't be resurrected.