Into the Mystery
Into the Mystery is one of the side-quests of Fallout 76 that involves you finding a dead body at Tygart water treatment plant only to ultimately reveal the secrets of the Riverside Manor. This will be a very eerie but knowledgeable quest.
After accepting the quest by investigating the dead body at the water plant, head to the Riverside Manor. Do note that investigating the dead body will complete the Misc. Quest “Search the Body” too.
Head inside, find the study terminal and access it for your convenience. You will find information on the Manor here. Finally, head to the northeast corner of the parlor room that is highlighted on the HUD.
Equip the ‘worn veil’ you found before in the quest, and a hidden entrance will become visible behind the black curtain. Follow this path and you will find a full-blown underground facility here. Investigate the area to complete this quest.