Zakal was a Vulcan mind-lord, holding the title of High Master of Kolinahr thousands of years ago, prior to the Vulcan Reformation. He was also known as Zakal the Terrible and Mind-Lord of ShanaiKahr, among other titles. The name "Zakal" means "the Fury, the Desert Storm", referring to the storm that occurred when he was born. He possessed tremendous psionic power, and was known for using his powers to melt his enemies into puddles at his feet.
Zakal was an opponent to the teachings of Surak, and he and his followers worked to sabotage Surak's attempts to bring logic and peace to Vulcan from their secret hideaway in the Mountains of Gol. Zakal condemned Surak's utopian vision as folly, believing that the old ways were necessary for Vulcan's survival. He believed that the people would come to their senses and rise against Surak, just as S'task - Surak's closest disciple - had. However, shortly before Zakal's death in 140005 (Vulcan Old Date), S'task and 80,000 followers were preparing to leave Vulcan so that Surak could succeed; they would later become known as Rihannsu - "the Declared" - and form the Romulan Star Empire. (TOS novel: The Romulan Way)
Khoteth, Zakal's most favored student - almost like a son to the High Master - later turned against him and took the name "Sotek", becoming High Master of the Kolinahru in Zakal's place. Imprisoned, his mind-powers curbed by Sotek's Kolinahr initiates, Zakal later contracted lunglock fever and was left to die in imprisonment. However, as a promise to his old mentor, Sotek preserved Zakal's katra, where he would use his mentor's knowledge to defeat enemy Kolinahru, led by Nortakh, another of Zakal's former students. Zakal was at first adamant that his knowledge and powers not be used to aid Surak, and swore that - if not for his imprisonment - he would have convinced S'task to stay and fight, even kill Surak himself if necessary. Though the thought of being used to advance the aims of a man he condemned as a coward repelled him, he was too vain to allow his mind to fade into oblivion. Thus, Zakal allowed Sotek to contain his mind within a vre'katra, which would later be preserved in the Hall of Ancient Thought at Gol at Sotek's insistence, despite the objections of other Kolinahru. In his dying thoughts, Zakal swore revenge on Surak and all who followed him, even if he had to wait for ten thousand seasons.
Fifteen thousand seasons (around two thousand years) later, in the Earth year 2271, a Kolinahr initiate named Sekar - believing Zakal's knowledge could restore Vulcan to greatness if it was brought to the Romulans - broke into the Hall of Ancient Thought and took Zakal's katra within himself, gaining the immense mind-powers and memories of the ancient High Master. With physical form at last, Zakal escaped from Vulcan by hijacking the scout ship of Dr. Keridwen Llewellyn, a human scientist studying the Kolinahru. The scout also had an unexpected passenger - Dr. Leonard McCoy. Sekar/Zakal took the scout first to the planet Arcturus to obtain a cloaking device to enter Romulan space, and then crossed into the Romulan Neutral Zone to meet with a Romulan Bird-of-Prey, keeping McCoy as a hostage (Zakal later admitted that he kept McCoy alive because "you are harmless and you amuse me", threatening to kill the doctor once he ceased to be so). However, instead of the welcome he expected, Sekar/Zakal and Dr. McCoy were both placed in the brig. Enraged, Zakal used his powers to kill the guards and - leaving McCoy in the brig - went to the bridge. After slaying the ship's commander as a demonstration of his powers, Zakal ordered the Bird-of-Prey to set course for Romulus.
Throughout the journey to the Neutral Zone, the stolen scout was tailed by a Vulcan scout ship piloted by Spock, and bearing both Dr. Llewellyn and High Master T'Sai. Also in pursuit - after McCoy tried to alert him of his kidnapping and of the clear insanity of his kidnapper - was Admiral James T. Kirk, who beamed onboard the Bird-of-Prey to rescue McCoy. T'Sai had carried Sotek's katra throughout the journey, until it was transferred to the esper-blind Dr. Llewellyn, who gained Sotek's powers but - because of her handicap - lost the ability to read Zakal's mind. With the Bird-of-Prey's self-destruct activated, Llewellyn/Sotek confronted Zakal on the bridge. Standing firm against the great sandstorm summoned by the evil High Master, Llewellyn/Sotek was critically injured when the shield protecting them from the sandstorm and Zakal's fire bolts was extended to protect Admiral Kirk and Dr. McCoy, who had gone to the bridge to rescue Dr. Llewellyn. After Kirk and McCoy were beamed to Spock's scout ship, the Bird-of-Prey exploded; with his last act, Sotek had condemned both himself and Zakal to oblivion, keeping the dangerous mind-powers out of the hands of the Romulans. (TOS - The Lost Years novel: The Lost Years)
Zakal also existed in the alternate timeline of The Tears of Eridanus. He was dismissed as a "deluded mystic who couldn't rhyme a couplet to save his life." (ST - Myriad Universes novel: The Tears of Eridanus)