An unstoppable beast in A Death Knight, in the sense that I use the term, is a melee fighter who raises his enemies to fight along side him (so basically a warrior/necromancer). The more enemies that surround you, the deadlier you become.

King Crusader armor is best described as a more armored Witch King from the Lord Of The Rings movies.

I'm trying to build a Death Knight commander like character, One of Skyrim's most powerful builds is the necromage vampire, to Molag about his creations, the Death Knight rose and charged to fight the mage.