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, at least 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. The quest branch can not be entered if the player is not of his or her original alignment, and the Bell of Opening must be obtained from the quest nemesis.

Therefore, if the converted player is not in the quest branch at the time of conversion and does not yet have the Bell, the game is generally unwinnable at this point. A player's alignment can only be converted once by means of an altar, and the quest leader will "see through" a helm of opposite alignment. There is a slight exception if the player has previously entered the quest, is neutral, and is not a wizard: the player can wish for the Eye of the Aethiopica to branchport to the quest (you have to have visited the quest once already because the Eye will not open a portal to a branch you haven't been to).

The best a player can do in an unwinnable situation is to escape the dungeon with the Candelabrum of Invocation, the Book of the Dead and any other sources of bragging rights.

Note that simply putting on a helm of opposite alignment does not render the game unwinnable--removing the helm (after uncursing, if necessary) restores the character's original alignment.