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. The caves are the only place where Cave horrors and Albino bats are found.
A light source is required in the caves, unless the player has lit a fire pit of Eternal Light near the entrance. In the north-east, two staircases ascend to small islands with teak and mahogany trees and seaweed spawns.
- Level 80 Cave horrors (Level 58 Slayer and a Witchwood icon needed to fight). They are scattered around the cave and are aggressive.
- Level 52 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 because they are aggressive.
- Cavey Davey guards the entrance to the caves. He warns players to be prepared before entering.
- Cabin Fever is required to access Mos Le'Harmless.
- 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 "Mos Le'Harmless" alludes to the Hitchhiker's Guide to the Galaxy series by Douglas Adams, in which the guide's entry on planet earth is simply "Mostly Harmless".
- 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.