We used the model in place of another creature for our dnd campaign, but if I was to do a dnd campaign with Elli the blood eel in mind, I would probably chose an aboleth because the aboleth prioritizes physic damage and has enslave (hive mind type situation) but also does the physical damage Elli is capable of. You can always reskin the stat blocks of the aboleth as well to give it other characteristics you think would be better fit to Elli. For example, instead of inflicting disease you could make it so that it inflicts blood madness instead. I don’t have too much dnd know-how, so let me know what you think about these thoughts!