Awaken, Dark Knight

First part gives us somewhat unsettling background for exploring locations like Death Palace, or Tomb of the Ancients. There's a constant tug-of-war between creepy and nostalgic vibe, that will keep you on your toes.
Technically, theme can simply progress layerwise with the player, the deeper you go (like in the recording), but layers can be mostly mixed-and-matched as needed.

Second part is full on "messed around and found out" moment. You stumbled upon the Death Knight's resting place and disturbed his slumber. Now you have to face the consequences.
I don't see it as a "boss battle music", arrangement is too busy for that and would tire the player out quickly. It's rather music for a cutscene right before the battle. Imagine dusty bones coming together, filling you with increasing sense of dread, untill the Death Knight in his final form stands before you, right on the final cadence of the piece.

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SoundCloud link:
Awaken, Dark Knight

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Done using Cockos Reaper (with some of its stock plugins), Darksichord 3, C.Hackl Church Organ, EZ Drummer, Native Instruments String Essentials, Ashlight, Rise&Hit.