Phaia was a true believer of Szass Tam. She was also opportunistic and self-obsessed like her master, even ready to betray him to save her own life. Phaia believed that no magic was too dangerous and no experiment too dark if it provided power for herself.
Although not easy, she can be convinced to betray Tam in exchange for her life. She can give the players some useful information and even possibly resurrect Kelson when he eventually dies. In that outcome, she readily escapes from Doomvault.