Unwinnable

While any given game of Nethack may be very difficult to complete, it is generally understood that every game is winnable - that is, no game is created that the player cannot win.

There is, however, one way that a game may be rendered unwinnable. If the player unsuccesfully performs an altar conversion by sacrificing at a cross-aligned altar with negative alignment, he or she is converted to the alignment of the altar. If this happens before the player has completed the quest, the game is unwinnable. The quest must be completed to get the Bell of Opening, but cannot be entered if the player is not of his or her original alignment. A player's alignment can only be converted once, and the quest leader will "see through" a helm of opposite alignment. Thus having one's alignment changed via altar conversion before completing the quest yields an unwinnable game.

The best a player can do in such a situation is escape the dungeon with the Candelabrum of Invocation and the [Book of the Dead]].