More on above spoiler.
Spoiler: showSecond and last boss fight is 3 knights and a wizard. You need to kill a knight to get a buff to damage the wizard. Killing all three knights grants extra stacks so you deal more damage. The buff has a countdown and you must cleanse in the burner in the center before the timer runs out. A crystal will spawn if after the knights die or if you take too long which shields the knights. Dropped another powerful reward. There were 90 seconds after the boss died. Maybe there's some secrets in there?