Giant Evil Slaughterfish:

If you've fought a Slaughterfish, then you know that it isn't pleasant. Somewhere in the game (I'm said to say I can't remember where I was), near a sewer (I think) you swim out into the sea with either a waterbreathing spell or potion or as an argonian and swim out around and deep until you the fish. Keep in mind, this may be a difficult fight as it is a monstrous fish, but in the end you get really good enchanted items based off of your level.
P.S.-You can make it easier by going into options and decreasing the difficulty level. I you guys can find him.