An illithid colony needs to unleash a serious bruiser on the party. Seriously tough beast could the mindflayers have captured and brain washed to use as an emergency weapon?

Most of the the typical mindflayer minions, quagoths, kou-toa etc, do not stand up to a large party of lvl 7 heroes. What sort of beasty could the illithid keep in reserve in their colony to fend off invaders?