Dragon Age: Origins
After collapsing on the Hillside Path, Wynne reveals to the Warden that she has died while fighting a powerful demon during Uldred's coup in the Circle Tower. At that moment, however, a benevolent Fade spirit (who, she believes, has been watching over her all her life) entered her body, effectively possessing and resurrecting her.
Throughout the game, Wynne believes that the Spirit is weakening and that her time is therefore very limited. Although the spirit never communicates with her intelligibly, she comes to believe that it gave her a second chance to both correct her past mistakes and accomplish one last great feat. She assumes that the purpose for which she has been returned to life is to help the Warden stop the Fifth Blight, motivating her to join the Warden's quest.Wynne presumably draws upon the power of the Spirit of Faith to use her unique ability Vessel of the Spirit.
Dragon Age: Asunder
Despite Wynne's assumptions, the Spirit of Faith manages to keep her alive for nine years following the Battle of Denerim. When Cole meets Wynne in the Fade, he is able to sense the spirit but doesn't realize what it is. In Pharamond's nightmare, the spirit manifests itself and helps Wynne defeat the demon.
During the battle of the White Spire, Wynne realizes that the reason she was returned from the dead was, all along, to resurrect the templar Evangeline de Brassard, who dies protecting her own son, Rhys. After Wynne transfers the Spirit to Evangeline, she dies on the spot but Evangeline miraculously comes back to life.As of 9:40 Dragon, the Spirit of Faith probably inhabits Evangeline's body, although its motivations remain unclear.