The Neural Inhibitor is a piece of wetware designed by Raven Micro-Cybernetics for a project funded by Arasaka. At the time Torrent was a Raven employee. He stumbled upon the project, and for reasons known only to him, he stole the plans for it, and then wiped Raven’s records clean.
Arasaka searched for him for many years, before finally capturing and imprisoning him. When the team went to rescue him he revealed that he had built the prototype and had it implanted in his own head. At that point, Torrent was brain dead, and functioned only as an AI construct. He ask the team to destroy his body which they did. He never did reveal exactly what the device did.