The sixth elder god was first mentioned by Zaros during The Light Within, stating that it is not of form, but 'something else'. He discovered this by searching for the memories of the Elder Gods within the Elder Halls on Freneskae.
At RuneFest 2017, the 'something else' term was used again in reference cosmology. The Elder Gods created the following worlds alongside a currently unknown 'something else'.
||Bik and something else
||Wen and something else
||Jas and something else
||Bik and something else
||Jas, Wen and something else
||Jas, Bik and something else
Interestingly, Zaros used similar phrasing when describing the power of Raksha, a shadow anima infused colossus. He said that the power of Raksha did not come from the Elder Gods but from 'somewhere else...'. This is despite the fact Raksha was empowered by the Codex, an Elder Artefact created by the Elder Gods. This could imply that the Sixth Elder God is related to shadow anima. Further, in Desperate Measures, Charos tells the player that their soul contains shadow anima, referring to it as 'something else'. Charos believes this was a consequence of Guthix anointing the player as World Guardian. For some reason, Guthix did not just transfer some of his divine power, but shadow anima too. He also believes that the shadow anima within the soul of the World Guardian could explain their resistance to the power of the gods.  However, it is currently unknown how Guthix became familiar with, or acquired, shadow anima.
In Heart of Stone, the World Guardian and Ariane find an empty Kra chamber beneath the Lumbridge Catacombs. This concerned Azzanadra as the chamber of Jas, Ful, Wen and Bik had already been discovered, and Mah never arrived upon Gielinor. However, he refuses to speculate or give any further information. As each of the Elder Gods have a Kra chamber, it is possible that this is the chamber of the Sixth Elder God.
- ^ Zaros, "The Light Within", RuneScape. "There is a chamber in the heart of this world. Within it lie the remnants of others, like Mah ... but more so. It was easy enough to find the memories, when you look carefully enough. Six of them, five of form and one of ... something else. The Elder Gods."
- ^ RRuneScape Lore Marathon - from RuneFest 2017. YouTube. 28 September 2017.*
- ^ Zaros, "Raksha, the Shadow Colossus (miniquest)", RuneScape. "It was not just a colossus, but a shadow colossus. Its power came not from the elder gods but from somewhere... else."
- ^ Zaros, "Raksha, the Shadow Colossus (miniquest)", RuneScape. "A handful of elder artefacts had been discarded by the Elder Gods. One of these was the Codex, a repository of both knowledge and power. When a great beast came across the strange monolith, it couldn't know what awaited. It emerged from the Codex fundamentally changed."
- ^ Charos, "Desperate Measures", RuneScape. "Well, you remember earlier when I spoke about souls. About how they're containers of anima that shape the anima with our experiences? Yes well. It appears that your soul is...different. It looks like when Guthix made you the World Guardian, by giving you some of his divine power, he also gave you something else. I believe the dragonkin called it 'shadow anima'. Your soul contains a carefully constructed enchantment of that shadow anima."
- ^ Charos, "Desperate Measures", RuneScape. "Well, on the plus side, it goes a long way to explaining your resistance to the power of the gods. As for all their terrible might, the power of the gods are ultimately applications of anima. This shadow anima, your shadow soul if I may, would be repellant to this energy shielding you from their influence. So, that's the good news."
- ^ Azzanadra, "Heart of Stone", RuneScape. "But Mah never made it to Gielinor... Unless...No, best not to leap to speculation just yet. So now we have a starting point, something that may help us find the Elder Halls. Did you learn anything else?"