Jas

Jas is believed to be an elder god, who existed even before Guthix had arrived and shaped Gielinor. Whether Jas created Gielinor is unknown, but he/she probably left with the other Elder Gods. The only thing that links is back to him/her is the Stone of Jas, a strong magical object that illustrates the great might the Elder Gods have, even surpassing Guthix's power.

The main reason Jas is believed to be a person instead of a place or material are the words spoken by the Oracle: Jas left a stone behind.