Mos Le'Harmless is a pirate base on the island of the same name. In French Mos Le'Harmless means Mostly Harmless. The city/island can only be accessed through completing Cabin Fever.

Barrelchest Anchor:  To get the anchor you must complete Cabin Fever and to repair/obtain a new one you need to head to the Smith.

Trouble Brewing: This is a minigame based on brewing the alcoholic drink rum. Teams must work together to brew the rum.

Cave Horrors: These are a popular slayer assignment and this is the only place to kill them. Outside of the city and in the Mos Le'Harmless Caves they reside.

Snakes: These are non poisonous and have a 100% drop of snake hide.
 

Charter Ships: One of the places available by charter ship is Mos Le'Harmless.

Bill Teach: If you have completed Cabin Fever Bill Teach will take you to Mos Le'Harmless from Port Phasmatys and vice versa.

Mos Le'Harmless is a very new city and has no history besides its inception.

Cabin Fever

For pures this city has almost no resonance. Trouble Brewing is a fun minigame but not important, and unless your a major slayer pure then Cave Horrors do not matter either.