Mysterious force

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The much-maligned mysterious force is a random level teleportation effect that occurs one quarter of the time[1] when trying to climb stairs or using a cursed potion of gain level in Gehennom with the Amulet of Yendor. Lawful characters are dumped up to three levels below their current location; for neutral players it is up to two and for chaotic it is up to one. (This includes occasionally being teleported around the level, a kind of "zero level drop".) The mysterious force does not act when you go up from the Valley of the Dead to the Castle, or when you enter Vlad's tower using the staircase. If you are inside the Wizard's Tower, it will leave you on the same level if it would otherwise place you below the bottom of the tower.

Monsters (especially the Wizard of Yendor) may also be affected by the force when they try to flee upstairs with the amulet. Unlike the player, monsters will always be sent exactly one level down by the force and will be placed near the up staircase, and only if they aren't already in the three lowest levels of Gehennom.[2] Cursed potions of gain level will not trigger the mysterious force for monsters. These effects are summarized in the table below.

Despite a code comment in NetHack 3.4.3 implying that the probabilities of each distance other than -1 are equal, this is not actually the case; the code actually randomizes the distance to move a player twice, leading to the following distribution:

Level change Lawful Neutral Chaotic Monsters
​−1 75.00% 75.00% 75.00% 75.00%
0 6.25% 8.33% 12.50% 0
+1 11.45% 12.50% 12.50% 25.00%
+2 5.20% 4.17% 0 0
+3 2.08% 0 0 0
Avg. Change -0.470 -0.542 -0.625 -0.5

Later versions of NetHack did not change the formula, but did change the comments so that they described the formula correctly.

Messages

A mysterious force momentarily surrounds you...
You were affected by the mysterious force and did not move up a level.
As <monster> climbs the stairs, a mysterious force momentarily surrounds <him>...
A monster carrying the amulet was affected by the force and moved to the previous level.

Probabilities

How long does it take to escape the force? The following values are for a typical 22-level Gehennom. On average, Lawfuls take around 30% longer than Chaotics, and Neutrals take around 14% longer. Before the corrected probabilities were discovered, players used to believe that Lawfuls had a much harsher penalty when escaping Gehennom.

Lawful Neutral Chaotic
Average staircase climbs (approx) 45.2 39.8 34.9
Climbs for 90% chance of escape 63 53 43

Strategy

Since Gehennom contains many non-teleport levels, climbing upwards would be annoying even without the force. The player is advised to save a few cursed potions of gain level to bypass especially convoluted levels, such as the Asmodeus' Lair.

There's an advanced strategy to bypass a few levels in the lower Gehennom by taking the portal to the Wizard's Tower, which typically moves you a few levels upwards (plus, the force is less harsh inside the Tower). The troublesome part is escaping the Tower's top level: cursed gain level will not work, and the Amulet blocks levelporting. However, if you stoned the Wizard while obtaining the Book (as an unique item, the Book won't be trapped in the statue), you can now unstone him, toss him the Amulet, escape the level via levelporting, and meet the Wizard on the upstairs. He should teleport to you with the Amulet.

Since the force is kindest to chaotics, lawful or neutral characters may wish to put on a helm of opposite alignment to become chaotic so that they can avoid the worst of it.

Variants

SLASH'EM, SporkHack, UnNetHack, xNetHack, and SpliceHack have completely removed the mysterious force. UnNetHack, xNetHack, and SpliceHack compensate for this by making carrying the Amulet block all teleportation, including intra-level.

UnNetHack also includes an effect that makes the Amulet of Yendor teleport itself whenever it is dropped for any reason.

Other mysterious forces

A mysterious force is also cited as the reason that one cannot teleport on a non-teleport level,[3] or descend the staircase on the quest home level without first getting permission from your quest leader.[4]

On some rare occasions, the mysterious force may take a more active role:

  • While digging with a blessed pick axe on the Plane of Earth, a mysterious force may create a cave around you.[5]
  • If you're caught robbing a vault, and drop the gold on a square where the vault's walls used to be, a mysterious force will move it back into the vault when the walls are magically repaired.[6]


References

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