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. To start the mission, locate the Sawmill in the middle of the Soviet Installation and engage the Trinity Soldiers that have pinned down Nadia. It is completed by killing all of them and finding her hiding in one of the lockers. The next mission is Chasing Grandpa, which leads Lara to the Wicked Vale.