Saint Gregory's tomb, one of three ancient crypts, is a required destination in the game only if Geralt chooses the Order 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.
The tomb, also known as the "old seat of supreme and sacred inquisition" (scary sounding!), lies beneath the Striga's crypt. Here Geralt is met by cemetaurs, fleders and a basilisk on the way to his goal: Saint Gregory's litany. In the tomb chamber, there is also a place of power, as well as sewants and green mold, but nothing else (you can play with a swinging skeleton, but it has no inventory).