Mana vortex is a special building available in dungeon. It doubles the maximum amount of spell points a hero can have, and fills them to this new maximum. Only one hero per seven days (not refreshed at the beginning of the week) can double their spell points. Additionally, a garrisoned hero can receive this bonus. This means, that the mana vortex bonus will go to the garrisoned hero if the bonus is available and the player opens the town menu.
AI never builds mana vortices.