Where is the dungeon? I couldn't find it. Also how often do they drop the black masks? Thanks

one inv of monks i usually get 2 with black dhide

The Dungeon where you can find Cave Horrors is located in Mos'le Harmless.

You also need a witches necklace thingy from a slayer master to enter.

 

 

 

Most important thing to bring is a light source of course. They attack with a mix of mage and melee, so dragonhide mix is best, or just wear full guthans. There's also a couple of safe spots for range.

 

 

 

The black mask drop rate is something like 1:600 in my experience.