Relay Station is a location within Far Cry 4's Valley of the Yetis addon. It serves a base of operations for Ajay while in the valley. Here the player can purchase upgrades to help fortify the grounds. At night, waves of attackers from the valley cult will charge the area, forcing Ajay to defend as quickly as possible.
Valley of the Yetis
In the old days, the Relay Station used to be the house of a farmer who shepherded Yaks, but the Disciples, who were then only expedition soldiers of the Royal Army, commandeered the house from the the farmer and subsequently converted it into their base of operations. The Royal Army Expedition set up a radio antenna on top of the house in order to make reports to Yuma about their progress in finding "The Relic". After the soldiers became Cultists known as the Disciples, they blocked the radio signals with three different jamming antennae, as they originally suspected that the CIA was spying on them through their radios and later on wanted to cut off all connections with Kyrat.
Upon reaching the valley, Ajay conquered the Relay Station and killed the soldiers who were stationed there. Ajay tried using the radio there to get help, but cannot due to the signal jammers; thus Ajay is left no other choice other than to use the Relay Station as his base, which he must defend from the frequent assaults of the Disciples at night.