Ancient Spawn of Morgathla was implemented at 2017 winter update. In order to fight the boss you need to use a Strange Mallet on a golden gong. The Strange Mallet can be acquired by giving one Mallet Handle, Mallet Head and Mallet Pommel to the NPC Gnominimus. The itens are rare drops from warzones 4~6 bosses.
This boss seems to be strong and hard to kill while it is very hard to access. What is the best loot confirmed from him? There might be a very rare loot that is still unknown such as Hammer of Prophecy from The last Lore keeper.