Phoebus

Sole survivor of a long-extinct civilization who were almost entirely wiped out by Thalan's people. She fought against Thalan until they were the last two fighters to the extent that she was able to learn his strengths and weaknesses.

Phoebus survived in a stasis pod in the Kohal system long into old age, and was retrieved by Colonel John Sheppard's Puddle Jumper and taken back to Atlantis. Her consciousness, stored in the headrest of the pod, downloaded itself into Dr. Elizabeth Weir (who happened to be in close proximity).

Phoebus awoke in Atlantis's medical bay. Pretending to be benign, she told the Atlantis staff that she had a limited amount of time before her consciousness was erased from Weir, and that the other pod held her husband, Thalan. To drive her lie home, Phoebus faked Dr. Weir's personality (using her memories) and managed to convince Colonel Sheppard to allow Thalan, who was as old as she, to migrate into his body so that she could properly say goodbye. Sheppard agreed, but unfortunately, once Thalan had gained control, it was obvious that the two were the coldest of enemies. Phoebus and Thalan, now in Weir and Sheppard, went on a rampage through Atlantis in an attempt to annihilate each other for the vindication of their respective races.

When she had Sheppard/Thalan cornered, Phoebus used Dr. Weir's access codes to threaten to vent Haylon gas into the living quarters if Teyla Emmagan did not kill Sheppard, whom Phoebus could see via a monitor. Dr. Rodney McKay was able to lock Phoebus out of the system in time before she could do any damage, preventing Teyla from sacrificing the Colonel.

It may have been Thalan who had the honor of disabling Phoebus. When Weir/Phoebus had Teyla and Sheppard/Thalan cornered, Teyla armed Thalan with a Wraith stunner pistol -- knowing he would fire upon Weir either way -- which he used to take her down.

Phoebus survived partially through the night, before Weir's body convulsed, expelling the consciousness completely.