R'sev was a location on the planet Vulcan and during ancient times was the site of the Khal people. This period in Vulcan history was the site of a cultural awakening which was similar to Sarpeidon's Lakreo Valley or Earth's Tigris-Euphrates civilization.
In 2270, Spock's time displaced son Zar decided to return to his homeworld by way of the Guardian of Forever when he realized that his involved on Sarpeidon's history helped advance the civilization in its early history. Deciding to journey to the Lakreo Valley 5,000 years ago, the significance of the matter was highlighted by Spock with it being similar to the Human Tigris-Euphrates civilization or the Khal at R'sev on Vulcan. (TOS - The Yesterday Saga novel: Yesterday's Son)