You will receive this mission only after completing [Video Ergo Sum], a cutscene will activate on the road (M26:14).
You will have a vision, in which one of the Professors and his student are searching for a cave entrance. It can be found nearby, go down the stairs to the [Holy Cave] (M26:15). Picklock the door. Once inside, you will be attacked by Stingers. Get rid of all the monsters to open the gate and witness another vision (M31:1). You will learn that you have to pass three trials. Before you move on (left), turn right and get the old parchment piece from the coffin (M31:2). After going along the corridor a bit (M31:3) you will pass the first trial, but your health level will drop near zero.
Wait for your health to regenerate or use a healing potion before moving on. You will be attacked by Necrises. Clear your way to the second trial (M31:4), where you will witness another vision. Before you, there's a set of platforms (M31:5), which you have to cross in a proper order. The old parchment piece will be useful here. There's a snake drawn on it - it shows the proper road through the platforms. The screen above shows the proper path (from the side you came from).
Head to the third and last trial (M31:6) - the trial of intelligence. The Dark Knight (M31:7) will let you through if you answer three questions.
You can refuse and fight the Dark Knight, however you have to remember that he's a very demanding enemy. Unfortunately you'll have to fight him if you give a false answer as well.
The Knight will give you three riddles to solve. Below are the answers:
1. Answer: Coffin
2. Answer: Star
3. Answer: Towel
If you manage to defeat the knight or answer properly to the question, you will be able to pass. Go to the place which the Professor with his student were searching for (M31:8). Search the sarcophagus and you will find the Leaky Grail.