Gehennom is a dungeon branch beneath the Castle, consisting of 20-24 total levels. The Valley of the Dead is, strictly speaking, the top level of Gehennom, although the game provides somewhat conflicting information on this, in that the Valley contains a downstairs that is identified as the gateway to Gehennom. Perhaps it is best to consider the Valley a kind of vestibule or entry-hall of Gehennom proper.
Gehennom has been in the game since NetHack 3.1.0. Prior to this, the underworld was Hell. Although it is no longer an instadeath to enter the underworld without fire resistance, you should still strongly consider it, since Gehennom is one of the areas where fire traps can be randomly generated.
Gehennom is full of mazes; it contains the lairs of several demon lords and princes, Orcus Town, and also the stairs to Vlad's Tower and the Wizard's Tower, as well as a couple of Fake Wizard's Towers. Other than these, there are no special rooms in Gehennom. The second level of Gehennom from the bottom contains the vibrating square. When the player performs the invocation ritual on the vibrating square it creates a downwards staircase to Moloch's Sanctum, which is the bottom-most level of Gehennom and the dungeon as a whole. Performing the ritual is the only way to gain access to the Sanctum level, although it is not necessary to use the stairs to reach it once the ritual has been performed--level teleport is also a viable route.
Specifically, Gehennom contains the following special levels (drawn from Dylan O'Donnell's Nethack Gazetteer):
- Valley of the Dead (1st level of Gehennom (immediately below the Castle), Dungeon Level 26-30)
- Asmodeus' Lair (2nd to 7th level of Gehennom, DL 27 to DL 36)
- Juiblex's swamp (4th to 7th level of Gehennom, DL 29 to DL 36)
- Baalzebub's Lair (6th to 9th level of Gehennom, DL 31 to DL 38)
- Orcus-town (10th to 15th level of Gehennom, DL 35 to DL 44)
- Wizard's Tower (three consecutive levels, 11th to 18th levels of Gehennom, DL 36 to DL 47)
- Fake Wizard's Towers (two separate levels, 3rd to 6th levels from bottom, DL 40 to DL 51)
- Vibrating Square (2nd level from bottom, DL 44 to DL 52)
- Moloch's Sanctum (Bottom level, DL 45 to DL 53)
Each of these levels will be present in every game, although the precise order in which they are encountered can vary. For example, Juiblex' Swamp might appear on DL 29, and Asmodeus' Lair on DL 31. See Mazes of Menace for a schematic illustration of the possibilities.
Additionally, there will be an upstairs to Vlad's Tower, which is not part of Gehennom, somewhere between levels 9 and 13 of Gehennom (DL 34 to 42). This upstairs can be included on any of the above-listed special levels, if they occur at the appropriate depths.
All levels of Gehennom that are not special levels from the above list are ordinary maze levels.
Many of the monsters found in Gehennom are resistant to fire, but not to ice, so Frost Brand is a popular weapon for this area. As there are a great many demons here, silver sabers and Demonbane can be quite effective, and Grayswandir is almost certainly the most effective weapon here.
Monsters found in Gehennom are:
Maze levels are tedious and time-consuming to explore manually, and this has given Gehennom a reputation as being a boring phase of the game. See Gehennom mapping for suggested mapping strategies.
Praying in Gehennom
A prayer in Gehennom is considered a prayer to Moloch, which can anger your god, amongst other things. It will never result in any of the beneficial prayer outcomes (such as healing you or uncursing an item). One consequence of this is that it is generally considered unsafe to use two-handed weapons while in Gehennom — if the weapon becomes cursed, you will not be able to pray to your god, and you will not have a hand free to get any of the usual curse-removing items out of a bag, while carrying a scroll of remove curse or potion of holy water in open inventory is risky because of the fire traps.
In SLASH'EM, Gehennom consists of only 17 levels, but this includes lairs for all of the unique demons.
- 1st level: Valley of the Dead
- 2nd through 6th levels: Four of these five levels are demon lairs, one each for Juiblex, Yeenoghu, Orcus, Demogorgon.
- 7th through 9th levels: Wizard's Tower
- 10th through 15th levels: Four of these six levels are devil lairs, one each for Baalzebub, Geryon, Dispater, Asmodeus. The other two levels contain Fake Wizard's Towers.
- 16th level: vibrating square
- 17th level: Moloch's Sanctum
The most infamous fact about Gehennom in SLASH'EM may be how Demogorgon's lair is always above and before Asmodeus's lair! Demogorgon is the most dangerous opponent while Asmodeus is typically the easiest opponent.
- NetHack Gazetteer: Gehennom describes the layout of Gehennom in vanilla NetHack.