The Crypt is a 1x3 region of the Dungeon found in the three western cells of 30 meters. It can be reached by a moss-covered stairway at the northwest corner of 20 meters. The stairway is not connected to any of the rooms of that map cell, so bring some cartography to find it and a digging tool. Within the Crypt itself, all the rooms have sealed doors, so you'll need to continue using that digging tool.
The main reason to visit the Crypt is that the southern-most map cell contains a random artifact.
All monsters in the Crypt are undead. As such, a wand of deanimation is exceptionally useful and can often kill multiple creatures at once. It's also helpful to bring a few potions of rejuvenation to recover from wight-induced strength loss, and plenty of blessed curing.