The material to make it is a rare ore that RS gold may only be found submerged. Get your swimming equipment and have Murphy bring you submerged. Close to the crabs is a crack in the stone that you can go through (50 Agility demanded ) to get to an underwater mine that has the ore that you want. Mine about 25 of these and then head back to Willus. He gives you a Magical Chisel. You need 60 Defence to wear it.
Speak to Willus and tell him that you've got the armor and he'll provide you a special book. Equip the armor and leap in the water in which the creature went. Now you are OSRS buy gold underwater and the creature is right before you. He is level 200 and utilizes melee strikes and an occasional magical strike.