Zar was a Human/Vulcan/Sarpeidonian hybrid introduced in the Star Trek novel Yesterday's Son. He was the son of Spock and Zarabeth, and had become a powerful leader on Sarpeidon about 5,000 years before the planet's destruction.
When Spock and Leonard McCoy were accidentally sent back in time to Sarpeidon's past, the pair met Zarabeth, who had been exiled to the past by the tyrannical ruler of Sarpeidon. Spock and Zarabeth fell in love with each other, and consummated their relationship leading to the conception of Zar.
Zar was born a few months after Spock and McCoy returned to the 23rd century. Zar was born with a Vulcanoid appearance and inherited his father's telepathic abilities. Zarabeth made sure that Zar received a proper education and told her son that his father was a loving and caring man who would someday return to take him to the 23rd century. For 17 years she raised Zar until she died in a fall. Not long after Spock, James T. Kirk, and Dr. McCoy used the Guardian of Forever to return to Sarpeidon's past where they offered to bring Zar to the future, an offer which Zar accepted.
Life in the 23rd century was challenging for Zar. Spock attempted to teach him the Vulcan ways but due to his upbringing Zar found it difficult to adapt. After helping defeat a Romulan attack on the Guardian of Forever, Zar decided that his home was in Sarpeidon's past. Historical evidence was also discovered that showed Zar had an important role to play in Sarpeidon's history. After a mind meld with his father, he returned to Sarpeidon, but had the Guardian place him in a more settled area called the Lakreo Valley.
Upon his return to the past, Zar married a young woman named Araen. The two conceived a child, however Araen died in childbirth and their child not long afterwards. This led Zar to believe he was incapable of fathering children, even though Dr. McCoy had assured him otherwise. Zar then became embroiled in the wars between various tribes on Sarpeidon. Having earlier swiped Spock's tricorder and taken it back into the past with him, Zar was able to use information stored in the tricorder to improve weapons technology and to listen to music - which was his way of escaping the brutality of the wars. After several years of non-stop fighting, Zar had a pronounced limp and other poorly healed wounds that had hobbled him.
Zar was later convinced by his father to return to the 23rd century to help heal the Guardian of Forever, which had malfunctioned and was causing severe temporal distortions. Zar spent several weeks in the 23rd century. During this time his injuries were healed by a Vulcan healer. He also noted how much the Enterprise had changed since his previous visit, and how he missed the red turbolift doors. Zar inquired about his grandparents, and Spock told him that both Sarek and Amanda would like to meet him.
After Zar healed the Guardian, Zar again traveled back to Sarpeidon's past, taking some Vulcan weapons with him. Defeating a tyrant known as Rorgan Death-Hand in single combat, Zar was able to bring peace to the Lakreo Valley and step into his role as one of the early great leaders on Sarpeidon.