The Barrelchest Boss is a level 170 NPC which you must kill either with mage or ranged, however, you can get him all the way down to 1hp with melee, but it will not let you kill him until you cast a spell to finish him off. You can get to him by teleing through your spell book: "Boss Teleport" > "Barrelchest."
People kill the barrelchest for the infamous Barrelchest Anchor. Nowadays, the Barrelchest Boss is constantly slain for its "Flipper" drop which is worth 100m - 200m