The Eris Coin is located at co-ordinates B-2 in the Cavern of the Gods, in the secret passage underneath the Pillars of Shadow, between the Altars of Isis and Horus. The coin only appears after re-opening the temple. The "blip", located in the southern section of the co-ordinate, also has 2-3 Cinderella Sea Slugs inhabiting it. This is one of the ten coins needed to complete the Solar System coin-collecting quest.