Star Trek: 10 Things You Didn’t Know About The Guardian Of Forever
4. Spock's Family Ties To The Guardian
In the novel Yesterday's Son, by A.C. Crispin, the Guardian is again under investigation by Starfleet. However, there is a complication - Spock, having discovered evidence that he may have fathered a child on Sarpeidon, 5,000 years previously (referencing the episode All Our Yesterdays), has petitioned, and been granted permission, to use the Guardian to travel back in time.
There, he discovers Zar - a child he conceived with Zarabeth, the woman with whom he fell in love in the past. Due to a miscalculation, Spock meets his son as a man, as opposed to a boy. They return via the Guardian, and only then discover that the Romulans have become aware of the existence of the device.
While the Enterprise battles the Romulans over the planet Gateway, Zar elects to return to his own time, knowing that he is crucial to the development of Sarpeidon's future. He shares a mind-meld with his father, and steps back through the Guardian, into the past.