Cave horrors drop the black mask, worth , at a believed 1/512 rate. They are located in the Mos Le'Harmless Caves, which require Cabin Fever to be completed. You'll also need a light source and a witchwood icon to avoid taking a great deal of damage. The easiest way to get to the caves would be either taking the fairy ring (requires Fairy Tale III - Orks Rift), or taking Bill Teach's boat to Mos Le'Harmless and travelling east from the town.