I could see Seith using some normal creatures he has rendered and animated and re-skinning it with some wild design, or just making it glowy/ghostly as a spectral version of some long dead battle general or something. If you stumble across a grave marker for them (fairly innocuous but if you are observant, you might see it) you have a chance to place some memorial items on the spot to try and rest their soul. Sprinkle hints of the boss's past amongst npc's dialogue and you'd know the items you may need to gather. A certain type of flower, a type of weapon, a pocket watch, a strip of fur, etc etc.
Placing these on the grave summons the ghost, who is still tormented by falling in battle or something and you have to either survive the fight (lots of clever dodging and moving) until the ghost calms down, or you have to find a way to wear them down with a weapon or item long enough to talk to them and explain things so they can rest in peace.

Not essential story, but depending on how "hidden" it is, could be a really fun "find" for people.
What do you folk think? do you remember hidden goodies in a game you managed to discover? What similar hidden boss or content might be fun to have implemented?
