The Guthix cape is a reward from The Mage Arena miniquest. To receive the cape, the player must defeat Kolodion in his four guises, then speak to him, step through the nearby sparkling pool, and finally pray at the statue of Guthix. Upon praying at the statue, the Guthix cape will appear next to the player and can be picked up.
The Guthix cape and the Guthix staff are required to benefit from the Charge spell, which enables the player to hit up to 50% more damage with the Claws of Guthix spell.
Once lost, a player can get a new cape by once again praying at the statue of Guthix. Kolodion does not have to be fought again. Alternatively, players can purchase one from Perdu for 23,000 coins (although this requires the player obtain the cape at least once from the Mage Arena).