This is likely pretty old information but Demons Souls and Dark Souls are two of my favorite games ever and if there are any other fans here I thought I'd share this.
Fans have been doing an amazing job figuring out all the hidden meaning behind the lore, and making theory as to how everything fits together. It's this sorta thing that makes me appreciate FROM's unique style of story telling as this was totally intended. But fans thanks to the PC version have been finding some really cool unused content, from unused parts of the script to recorded voice overs that never made it into the game for some reason or another. A whole covenant and several NPC related quests have been found to say nothing of unused weapons, armor and items.
Cut enemies or prototypes of other enemies like the Rockworms, Marvelous Chester, or the completely cut Spider Hollow are really cool to see put back into the game by the modders as they test how close they were to being finished. Lately I found this video that shows a completely unused boss.
Undead King Jar Eel
The common belief is the player would encounter this boss in New Londo and he would have been one of the four kings who in the final game are the main boss of that location. Perhaps the four kings were a later concept and Jar Eel would guard the key to the Abyss.
His character model is basically finished minus some incomplete textures and effects, his cape physics are still kinda glitchy too. He has animations for all his attacks, various sword swipes movement, and what seems to be a variation of the Life Drain attack used by the Dark Wraiths or players using the Dark Hand weapon/shield. His audio is likely a place holder as it's the same as False King Allant from Demons Souls. The up loader simply replaced Lord Gwyn with Jar El for testing purposes as the two share some similar patterns.
Apparently the size of the boss made testing the effects of his attacks challenging, but the ones that do connect don't seem to have any damage values yet.
[youtube]http://www.youtube.com/watch?v=Z4-boABnb_U[/youtube]
It'll be interesting to see if any of Dark Souls' cut content makes it into Dark Souls 2 in some form or another. The DLC expansion "Artorias of the Abyss" was a lot of their scrapped ideas involving the Kingdom of Oolacile and the Knight Artorias who is mentioned a few times in the base game but never really explored. Was one of the few examples of worth while DLC for a game.