Mindblade Spectres are ghosts that appear in the Chaos Planes of the Underworld and also during the quest Escort of Souls. Their high health, high armor rating against physical damage, and multiple interrupts supported by Migraine make them a significant threat to both spellcasters and martial classes. Protecting the Lost Souls from the Mindblade Spectres is vital for the party, since their death will result in quest failure and will kick the whole party from the Underworld.
Mindblade Spectres also appear after destroying a Banished Dream Rider. A group of three Mindblade Spectres spawn, after which a second group of six Mindblade Spectres and a third group of nine Mindblade Spectres spawn, each after the previous group has been destroyed. Your party should be aware of this before confronting any Banished Dream Riders or the fight might result in a party wipe.