The ancient crypt beneath the old mine, one of three such crypts, is a required destination in the game only if Geralt choses the Scoia'tael path. The crypt remains accessible for other path choices, but is not required to complete any quests. It can be explored purely for the sake of interest, XP and loot.
As the name suggests, this ancient crypt is hidden inside the old mine in the swamp cemetery. Inside, Geralt is met by alps, bruxae, wraiths, cemetaurs and fleders on the way to his goal: a Bifunctional fiber and alloy recirculator, which is found in a dwarven trunk. There are also sewants and green mold.