The Holy Challenge of Combat was an ancient Vulcan ritual engaged in by the priest-kings of the Te-Vikram Brotherhood in the days before the Age of Surak. It was a hand-to-hand fight to the death, without weapons, to determine moral righteousness.
In 2296, Vulcan dissident Sered issued this challenge to Spock in the caves of Obsidian. Spock was able to incapacitate Sered, and have him returned to Vulcan. (TOS novel: Vulcan's Forge)