Bring the heart to Willus to complete the creature. Now a cutscene will play and the creature will crack down a castle wall and dive to OSRS gold the ocean where you can't access to it. Yet. Talk to Willus and he will inform you about a magic armor known as"Merma Armor" that allows one to breathe underwater. The material to create it is a rare ore which can only be found underwater. Get your swimming gear and have Murphy bring you submerged. Near the crabs is a crack in the stone that you can go via (50 Agility demanded ) for an underwater mine which gets the ore you need. Mine about 25 of them and head back to Willus. He gives you a Magical Chisel. Melt the ore in a furnace and then use the Magical Chisel on the ore to create pieces of it. There are 3 pieces: the Merma Top, the Merma Leggings and also the Merma Mask. You want 60 Defence to utilize it. Speak to Willus and inform him that you have the armor and he'll give you a special book. Equip the armor and then jump into the water where the creature went. Now you are underwater and the monster is right in front of you. He is level 200 and cheap RuneScape gold uses melee strikes and an occasional magic attack.