Mind flayer tadpoles once implanted into the humanoid victim devour the hosts brain, taking its place and transforming the host’s body through Ceremorphosis into a mindflayer. Ready to serve the elder brain. However barbarians that walk the path of the ceremorph are the host of rare, mutated tadpole that has no connection or desire to serve an elder brain. It will not devour their brain but instead live in symbiosis with the its host brain. As long as the tadpole isn’t removed it will impart incredible psionic powers to the barbarian. However as the tadpoles powers grow so does the its hunger for the sweet nourishment of humanoid brains.
Art by Wayne Reynolds for Wizards of the Coast