The Serpent God is a deity that presumably exists in The Legend of the Golden Axe universe. Unlike Ax-Battler, Tyris-Flare and Gilius-Thunderhead who draw their magical power from the elements of Planet Earth, Cobraxis (and presumably Viprax along with other wizard priests) prays to the Serpent God to achieve certain abilities. Furthermore, Cobraxis doesn't always make this clear, seeming as if he conjures writhing snakes out of nowhere. In the final part of Plague of Serpents, the situation is looking bleak for Cobraxis, so the villain calls upon the Serpent God to turn him into his image. The transformation convinces Cobraxis that he is now a God and prepares to kill Queen Silpantia-Dragontamer, but the larger size makes him an easy target for Gilius-Thunderhead and the Golden Axe. Falling down a bottomless chasm in two halves on fire proved that this "God" wasn't so immortal after all.
The Serpent God seems to have some analogue with Goddess Flora, the deity that Yuki-Plantcharmer prays to to achieve her power.