Raw cave eels is a fish that require level 38 Fishing to catch as well as a Cooking level of 38 to cook; doing so turns it into a cave eel. They are used in several quests that require the gathering of objects and can also be used in the Creature Creation minigame to make a frogeel.
They can be caught from Lumbridge Caves with a fishing rod and bait. If raw cave eels are specifically being targeted, bring plenty of bait as Slimy eels are much more likely to be caught.
Cave eels or slimy eels are the primary ingredients in the popular Dorgeshuun dish eel sushi. The eel is presumably served raw and wrapped in swamp weed. However, you cannot make the dish yourself.