The Disturbance in the Sawmill
The Disturbance in the Sawmill is the first mission of the Rise of the Tomb Raider DLC Baba Yaga: The Temple of the Witch. Despite the mission taking place in the Soviet Installation, this mission is wrongly associated with the Siberian Wilderness region.
When you own the DLC, you will receive this mission automatically when returning to the sawmill area after escaping the Gulag. It is completed by killing all Trinity Soldiers and finding Nadia hiding in one of the lockers.