The Pact of Chieftans took place among the Chieftains of the Nords of Skyrim during the First Era.[1]
It occurred in 1E 420 after Olaf One-Eye became High King of Skyrim, which resulted in the end of the War of Succession.[2][1]
Before this had been concluded, though, the Cyrodiils were busy reclaiming the land that had been lost to them during the First Empire of the Nords.[3]