You must speak to Guthix, who it's located in his cave, enter once you have completen every quest. The cave is located in Ardougne. Guthix will give you the helmet for 5,000,000 coins. In the game this is the time in which you will meet Guthix. Helmet of legends was once worn by the Elder Gods, but now it's full power is long gone and has turned into a sturdy helmet. in which it can poison you're opponant when they attack you or recoil's damage to anything. When fighting opponents the helmet of legends will become very useful because of its abilities.