Commander Puretide was a legendary Tau military leader of the Fire Caste whose exploits and stratagems have influenced every subsequent generation of Tau commander, or Shas'o. Basically, he's the Tau Empire's Sun Tzu.
Puretide originally hailed from the Dal'yth sept, and led their armies to countless victories over the various opponents of, and resistors to, the Greater Good. He was a key figure in the Tau Empire's Second Sphere Expansion campaigns, and many of the Empire's Second Sphere worlds owe their very existence to the efforts of Commander Puretide. Severely wounded in battle near the end of his life, Puretide at long last retired and subsequently became a hermit. He decided to commit his vast knowledge of military command and strategy to the benefit of future generations in the hopes that they could emulate his balanced and effective style of warfare.
Many Fire Warriors studied under his tutelage before he died, and countless more have studied his writings and insights since. Few have been able to fully grasp his teachings and imitate his success on the battlefield. Most commanders took only one aspect of Puretide's instruction and built a school around it, the most prominent examples of this being the opposing battle philosophies (and political philosophies) of Commander Farsight and Commander Shadowsun, both of whom were pupils of Puretide. In an even bigger fit of nostalgia, the Tau have developed a piece of technology for use by commanders known as a Puretide engram neurochip, which is a microchip implanted in the brain that grants the Shas'o ready access to a measure of Commander Puretide's tactical genius. Crunch-wise, this is a signature system that allows a Character in a Crisis Suit to select one of five special rules (Counter-Attack, Furious Charge, Monster Hunter, Stubborn, or Tank Hunter) at the beginning of the Movement Phase to use that turn.
Though it should be noted that said neurochip came at the expense of Puretide's life, what's worse it was done by the hands of his own pupil, Farsight, who was forced to get it by the Ethereals. Notably Puretide's last words to Farsight before he died from the neurochip sucking up his brain was, "Don't trust them all, my child"; which were certainly words Farsight took to heart.