Pick up the item and then move two chairs with corpses onto nearby platforms. Moments later a third zombie approaches.
First hit him once with the fist and wait until he comes closer to the empty platform. Your second hit should move him there. If you failed for the first time, wait for another creature and repeat these actions.
If you succeed, the grate blocking the passage will be raised.
Quickly go to the other side, climb up and pull the nearby lever.
In the newly unlocked room you'll encounter three walking undeads. You have to use your crossbow and nail them to the opposite wall.
If you do not have bolts, jump to the other side and wait until trapdoor is launched. In the cell at the bottom you'll find ammo you need.
When you succeed with nailing all of the, quickly move to the crate with the violet stone and pick up the ring.
Thanks to that you'll open the gate in room of hangmen.
Behind it you'll find a bar, on which you have to use the zipline, and the ladder leading to the surface.
Once you leave the crypt, use the zipline again to get to another tomb.