Possession Spells allows a Keeper to directly take control of a Minion's every action. While possessing a minion a Keeper cannot be harmed by normal means, only through special spells. Mukrezar used Cranium Internum, which is a more rudimentary version which allows you to only control the limbs. A newer version is In Crania Positoris, which gives you full access to the creatures mind and allows you to speak even if the creature has no vocal cords in order to command your troops directly.
However, possession is not an intrinsic power to the dungeon heart.