The crystal will reveal the truth about how three of the tournament's combatants died. Take it with you to the graveyard east of the Tournament Grounds and use it before the graves of Sir Wendell Balfour, Lorien Sunblaze, and Conall Irongrip.
If indeed these men were murdered, then we must find a way to tie it to the Black Knight before he strikes again. Look closely at what the crystal tells you, <name>, for we must be certain these deaths were not accidental.
So you say Lorien Sunblaze was done in by poison and Sir Wendell was burnt alive? Dreadful ways to die, both of them. Only Conall was killed by cold steel, and from your description, it was a coward's strike, not honorable combat that laid him in his grave.
All three sparred with and were scheduled to fight the Black Knight. This cannot be a coincidence. Now, we must find a way to prove it was the Black Knight's work.