Open the final level of the tomb of the king under the mountain.
Start at the end of Erebor: Thrain's Tomb Level 3.
You will need to swim for this entire quest. You need to find a hole in one of the rooms and pass through it.
The eels here are not hard to kill, but can shock you and/or prevent you from moving. If an eel prevents you from moving you can simple retry the move and you will not lose any endurance. There is a random chance the eel will prevent you from moving Electric eel flanks you, and prevents you from swimming <direction>.
You cannot move immediately after swimming into another room, if you try you will be prevented from moving You can't swim that fast! but will still lose endurance.
Do these commands, one at a time, if you encounter an An electric eel in the last room, you can either kill it, or swim back to the entrance (reverse the commands exactly) and wait for the eel to move. Start this quest with full endurance and be unencumbered, you should wait a few seconds inbetween each swim as you will be prevented from swimming for a short period after entering the room You can't swim that fast!, but will still lose endurance if you try.
swim bottom
do swim bottom
swim e
swim e
swim se
Now you are in the correct room, if there is an eel here, either kill it, or swim back using the following commands (one by one, don't do them all in one go as you will be prevented but still lose endurance each time).
do swim nw,swim w,swim w,swim top,surface
Now, going back to the room, assuming the eel is now dead (or gone, or never appeared)
do move jar,pry plaster,enter hole
You have now entered the last level of the tomb.
If any of the information here is missing, or wrong, please let us know.