Kraken

The kraken,, is the most difficult of the sea monsters in NetHack. It possess the instadeath drowning attack, as with its lesser giant eel and electric eel brethren.

Kraken are sometimes a target of reverse genocide, as they are worth a high amount of experience points (even more than the Riders and below only that of some of the demon princes) and only have one hit point outside of their native environment.