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 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.
- Cabin Fever is required.
- Cavey Davey guards the entrance to the caves. He warns players to be prepared before entering.
- Little Cave of Horrors
- 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.
- Albino bats only drop Bat bones.
- 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'.