You’re one of the big DK people on here. There’s some interesting DK lore in the Bolvar dialogue.

I am excited to see who you fight when you actually go into Frostmourne.  My pet theory here is that at least one of the ‘powerful spirits’ inside the blade itself is none other than Ner’zhul. Since they did add him as another NPC, and he could be in the blade, given there was nothing about his spirit after his fate in the Arthas novel.