Altar

An altar, represented by an underscore, is a place where you may worship gods. An altar with an aligned priest in the same room is a temple.

Altars and priests are co-aligned if they are of the same alignment as you, cross-aligned if they are of a different alignment. An altar is aligned if it has lawful, neutral or chaotic status. Altars of Moloch are unaligned. Unaligned altars do not appear in the main dungeon (except as mimics), only in Gehennom and some quests.

Altars not located in temples are scattered randomly throughout the dungeons. The Valley of the Dead has an unaligned altar located in the temple of Moloch. Minetown is also guaranteed to have a temple altar (unless it was destroyed in a bones file).

Altars can be used to detect the BUC status of items. This is done by dropping the item onto the altar. A black flash marks a cursed item and an amber flash a blessed one. You can not detect the BUC status if you are blind. You can not detect whether an item is blessed or cursed if you are hallucinating, as you will not be able to distinguish between amber flashes and black flashes; you can however still identify uncursed items as there will not be a flash at all.

If you #sit on, engrave, or kick an altar, it will lower your luck. If the altar is of your alignment, it will reduce Wisdom by one point instead.

If you (or a monster) [z]ap a wand of digging downwards, you get the message "The digging ray is ineffective" and nothing further happens. However, if you break a wand of digging or apply a drum of earthquake while standing on an altar, it will be destroyed (unless it is an high altar).

Standing on an altar scares vampires as if you were standing on Elbereth or a scroll of scare monster. Any altar will have this effect, including an altar to Moloch.

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Making water holy/unholy
If you drop one or more potions of water onto an altar and #pray, the potions become blessed if the altar was aligned with you and become cursed if the altar was not aligned with you. This occurs regardless of whether the water was blessed or cursed before. Be aware that praying on a cross-aligned altar will anger your god, decrease your luck, and you might be punished in some other way.

Normal prayer timeouts apply when praying on altars.

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Encyclopedia entry
Altars are of three types: 1. In Temples. These are foOMFG WHY WONT IT DELETE DELETE DELETE

Variants
Players of NetHack brass and SLASH'EM need to be wary when converting altars, because of the increased chance that the altar's original god will dispatch a minion against you. At experience level 7 or above with high alignment, the chance of a minion appearing after converting the altar is the same as vanilla. As in vanilla, the god will yell, "Thou shalt pay for thy indiscretion!" Players with a high luck will have fewer minions dispatched against them.

Even worse, there's an additional chance of one or more minions appearing post-conversion. In SLASH'EM, if you are at experience level 4 or above, there is a chance that "You feel (god) is very angry at you!" and the god dispatches two minions. Afterwards, you still have the original chance of another minion, as above. Again, luck is a factor.

In NetHack brass, you will always receive "You feel (god) is very angry at you!" For all experience levels, at least one minion will appear. Two rolls are made: the first from 0 to 9, the second from 0 to 19. For each roll, if your level is less than the generated random number, a post-conversion minion is summoned. Luck is not a factor. Thus in brass it is very dangerous to convert altars. You might convert an altar at experience level one and have to flee to another level from one minion, or three minions if the game rolled 0 twice, thus giving you no use of the converted altar. If converting an altar at experience level 7 or above, you might still get the pre-conversion minion, forcing you to fight as many as four summoned minions.