Sutekh and Nephthys tricked Osiris into an airtight sarcophagus and sent it into space. His psychic powers were blocked so he could not project himself into another body. Isis searched for him but failed to find him before he died. However, Osiris managed to project enough of himself into the pilot of Isis' ship to create his psi-child Horus. (PROSE: The Sands of Time)
After the death of Osiris, Sutekh and Nephthys escaped to Earth, destroying Phaester Osiris along the way. She was with Horus when Sutekh was imprisoned in Egypt. (TV: Pyramids of Mars, PROSE: The Sands of Time)
She waited on Earth for the eventual awakening of Sutekh in a vault built by her followers, that was to function as a trap for Sutekh. However, it was taken over by followers of Sutekh. Without the Eye of Horus, however, they were not able to destroy her.
She was freed by Bernice Summerfield and Ace in Switzerland in 2015, guided there by an illusion created by Isis called Alozza (Arabic for Isis). She transported Bernice to ancient Egypt, to the time of Hatshepsut's reign. (AUDIO: The Vaults of Osiris)
Alyx Courtland, transported from the 21st century to the 29th century by the Doctor along with her parents, who were cultists of Sutekh, turned out to have had a part of Isis in her since birth. When she was killed by Sutekh, she came back to life fully as Isis herself. She stayed on Earth, which was by then devastated by solar flares, and used her powers to bring life back to the planet. (AUDIO: The Tears of Isis)