Ritual Skull Sea Of Thieves. Only one skull stash voyage can be purchased per month. At the cost of 30 doubloons, they are.
The process behind completing the fort of the damned is a bit more complicated than just dumping a ritual skull there. This is so pve lords can’t abuse the fotd on alliance servers. The skull stash voyage is a stash voyage sold at the bilge rats black market at every tavern in sea of thieves.