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 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

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

Variants

SLASH'EM, SporkHack, and UnNetHack have completely removed the mysterious force.

UnNetHack has, however, replaced the mysterious force with 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

  1. do.c in NetHack 3.4.3, line 979
  2. muse.c in NetHack 3.4.3, line 774
  3. tele in teleport.c
  4. do.c in NetHack 3.4.3, line 1018
  5. mkcavearea in dig.c
  6. wallify_vault in vault.c
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