A Sword of Puretide is a prototype T'au neurochip that served as the predecessor of what would become the Puretide Engram Neurochip. Developed during the midst of the Damocles Gulf Crusade by the Earth Caste at the request of the Ethereals, these experimental neurochip prototypes contained the recorded military strategies of the great T'au military leader Commander Puretide.
During the war, these "Swords of Puretide," as they came to be known, were surgically implanted into the minds of many Cadre Fireblades and T'au Commanders, allowing them to function as great military leaders that thought alike and could adapt to nearly any situation.
Implanted commanders enjoyed great success when facing off against rank and file Imperial troops, but suffered when the Imperium brought forth its psykers to battle. Once implanted with a Sword of Puretide neurochip, a commander could not draw upon any of their own previous strategic knowledge. This caused many to falter badly in the face of the psyker threat, with the confusion costing hundreds of lives as the minds of the T'au leaders struggled to respond.
After the war, all of the Puretide engrams were forcibly removed from the minds of those who had been implanted with them. These invasive procedures left those who were operated on as drooling and pliant simpletons, a sad loss to the T'au Empire, but such was the painful cost of victory in the eyes of the Water Caste.
A notable exception to this was Commander Shas'vastos, a close friend of Commander Farsight's, who was hidden away before his scheduled engram removal; thus becoming the sole survivor of the Sword of Puretide program.
Whilst the prototype neurochip suffered rapid degeneration quickly thereafter, the commander was secreted away in stasis until the Farsight Enclaves Earth Caste scientist O'Vesa found a cure many decades later by recalibrating the neurochip and allowing the warrior to be safely awoken.
- Farsight Enclaves - A Codex: Tau Empire Supplement (6th Edition) (Digital Edition), pp. 120-121, 129-130, 230, 250-251