The moment Nina raged, the tarrasque blood in her veins boiled. Barbarians are powerful fighters even without that skill, but using it means they are in clear danger and need to end the fight as soon as possible.
The blood picks up on that note, racing to protect the holder by shifting their biology to resemble that of the great mother, the primordial tarrasque.
The change starts with the brain. Having such a weakness that's essential for survival is a danger. The blood eliminates the brain and instead transforms the whole body's neural network into a cognitive system.
That resulted in a decentralized nervous system that stores information in any part of the body while providing sharp reaction time and instincts.
Even if the holder's body were to be blown apart, leaving only one toe, that toe's nerves are the brain, and they would have at least the bare minimum information to regenerate the whole body in a second.
