The twin dragons have a chance to attack the player at any point while they're walking across the valley's frozen lake or if they approach the Drain Vitality Word Wall. There is no specific quest objective to track down Naaslaarum and Volslaarum, though players may end up baited into this encounter if they happen to notice a certain Word Wall. During Touching the Sky, players travel to the hidden valley known as the Forgotten Vale, a unique location only available once Touching the Sky is started. Two twin dragons can be encountered during the Dawnguard DLC quest Touching the Sky, though they aren't actually part of the quest objectives themselves.
