There are many planes of existence between mortal human existence and true enlightenment. Millennia ago, in a distant galaxy, the Alterans were a human society on an evolutionary path to ascension, but a philosophical division grew. Those who left their galaxy and journeyed to the Milky Way came to be known as the Ancients, and those they left behind came to be known as the Ori. Both the Ancients and the Ori eventually evolved physically and mentally to the point that they were able to shed their physical bodies and live as energy on another, higher plane of existence. The Ori, who manifest themselves as fire, demand that they be worshipped by humans, and although they promise ascension to their followers, they instead draw their power from the faith of those who believe them to be gods. The Ancients, however, also known as the Others, believe fervently in free will and refuse to intervene in the affairs of those on the lower planes.
In their ascended state, the Ancients appear as an ethereal glowing light, however they can also remain invisible, or take on a human appearance in a non-corporeal form with limited ability to interact with objects in the physical world. They can choose to retake mortal human form, however it is impossible to ascend once again without the help of the Others. The Others possess extraordinary powers, including the ability to control the forces of nature and a kind of telepathic communication or exchange of spirit that shares one's innermost essence. However, ascension does not make one all-knowing or all-powerful. One gains enlightenment and a much greater understanding of the universe and all its knowledge, however that is merely the beginning of life's journey.
The ability to ascend is not limited to the Ancients. If one is pure of spirit and good to the very core, one can choose ascension and find the path to achieve true enlightenment. Although one should attain ascension on one's own, it is possible to be guided along the path, and Oma Desala has helped others, including humans, to ascend. In doing so, she has broken the Ancients' highest law, that no lone ascended being shall help a lower, or human, ascend, but the Others, for reasons of their own, permit her to walk the fine line of noninterference.
Among those who have ascended are the monk from the temple of Kheb, Orlin, Oma Desala, Merlin, Morgan Le Fay, Shifu, and the Abydonians killed by Anubis. Anubis himself had once ascended but had been cast out by the Others and remained as a form of energy, stuck somewhere between mortal existence and ascension. He attempted once more to achieve ascension through technology rather than spiritual enlightenment by creating Khalek, a highly evolved genetic hybrid. Research theorizes that 80 to 90% brain activity is required for conscious, willful ascension, and Khalek had nearly evolved to that level, but Anubis abruptly abandoned his experiment before completion.
As Daniel Jackson was dying of radiation poisoning, he was guided to ascension by Oma Desala, and became one of her followers. However, when Daniel attempted to intervene in human existence by confronting Anubis, he broke the most sacred rule of the Others, and Oma Desala drew him away. He was returned to human form with no memory of his time as an ascended being or of the Ancient knowledge he had acquired. When Daniel was killed by Replicator Carter, Oma Desala once again brought him to a plane between mortal existence and ascension to offer him a second chance to choose enlightenment over death. Unable to accept the restrictions that would prevent him from intervening against evil, Daniel once again chose to return to his mortal human form.
As the Ori prepared to invade this galaxy with the intention of destroying the Ancients, others among the ascended Ancients began to reconsider the law against noninterference. As an ascended Ancient, Orlin could have cured the Prior plague that threatened to destroy Earth, but unable to take action as an ascended being, he chose to take human form to offer his assistance in finding a cure. He had hoped that taking the form of a child would allow him to retain his Ancient knowledge longer, but instead the vast knowledge overwhelmed his mind, and he was unable to ascend again. Merlin had retaken human form in order to create a weapon capable of destroying the Ori, but the Others had tried to stop him, and Morgan Le Fay hid him away in stasis. Morgan herself attempted to guide SG-1 in their search for Merlin's weapon, but as she appeared to Daniel with the information he sought, she was drawn away by the Others.
Even after Merlin's weapon was found and sent to the Ori galaxy, there was no way to know if the ascended Ori had been destroyed, but as Adria, the human representative of the Ori in this galaxy, faced her own mortality, she prepared herself to shed her physical form, and she ascended.
Cross Reference: Abydonians, Adria, Alterans, Ancients, Anubis, Brain Scan Device, Daniel Jackson, Jim, Khalek, Kheb, Merlin, Monk, Morgan Le Fay, Oma Desala, Ori, Origin, Orlin, Others, Shifu, Skaara
Episode Reference: Maternal Instinct, Absolute Power, Ascension, Meridian, Abyss, The Changeling, Full Circle, Threads, Avalon, Origin, Prototype, The Fourth Horseman, The Pegasus Project, Dominion