|Release date||4 July 2006 (Update)|
|Main music||Little Cave of Horrors|
|Strongest monster||Unspeakable horror (level 105)|
|Dwarf multicannon allowed||Allowed|
|Quests taking place here||Cabin Fever|
The Mos Le'Harmless Caves, also called the "Cave of Horrors", are found on Mos Le'Harmless, which players can reach only after completing the Cabin Fever quest. A light source is required in the caves. In the north-east, two staircases ascend to small islands with teak and mahogany trees and seaweed spawns. The caves are the only place where Cave horrors and Albino bats are found.
- Level 77 Cave horrors (level 58 Slayer and a Witchwood icon needed to fight). They are scattered around the cave and are aggressive.
- Level 105 Unspeakable horror (level 58 Slayer and a Witchwood icon needed to fight). They are found on north-eastern part of the cave and are aggressive.
- Level 43 Albino bats. These are rarely fought because they have no drops, and mainly serve as an annoyance to players who wish to fight the horrors.
- Cabin Fever is required.
- Cavey Davey guards the entrance to the caves. He warns players to be prepared before entering.
- The music "Little Cave of Horrors" is a reference to the film Little Shop of Horrors.
- Cave horrors are regarded as having good drops. They occasionally drop prized Black masks, which enhance players' abilities on Slayer assignments and are used to make Slayer helmets.
- The "teak and mahogany" islands are rarely used for Woodcutting, given their distance from a bank and the need to cross the caves. Both regular Seaweed and Edible seaweed are found on the islands.
- The black masks appear to be a stone, with pieces carved away when compared to the markings littered on the floor around the dungeon.
- The name is a play on words, Mos Le'Harmless. Mostly Harmless which is a reference to the book, movie and radio show 'The Hitch Hiker's Guide to the Galaxy'.
- Mos Le'Harmless is also a map type in the Funorb Game Arcanists.
- When a player enters the Mos Le'Harmless Caves, they have two options before entering should they not have a Witchwood icon. The option for entering says, 'Yes, I am not afraid to be killed in nasty ways', a reference to the song 'Brave Sir Robin' in Monty Python and the Holy Grail.