The Gehenoth is the strongest monster in the game. If you are at a high level, it may appear when you set camp. In this case, flee, as the Gehenoth will probably be too strong for you at that point. During the final showdown, you must kill the Gehenoth to win.
The very first time you meet resident healer Eustacia in the Dawnstar camp, she will provide fair warning about this creature: "I must also warn you that outside these walls is a fearsome creature that has slaughtered any who fight it. It appears when you rest. Do not fight it, you are not ready. Run."
There are actually two variants of this creature; the weaker Gehenoth is the one who will attack you in your sleep while the much stronger Gehenoth Thriceborn is the last enemy you will encounter during the final invasion from the Ice Tribes. When the Gehenoth Thriceborn is dead, the game is successfully completed.
- While tempting, there is no reason to camp endlessly to spawn multiple Gehenoths. They will only drop a piece of random loot, just like all other creatures in the game.