Seanna is an oracle of Morrighan, initially found in the Sewers under Rocheste whom Verafim and Shadow Shaman Ingkara were casting some sort of spell until interrupted. Laurys proclaims that she is the prophesied oracle that will lead the way to Erinn. She stays in the Cathedral for now. Little else is currently known about her.
After completion of Episode 10, Seanna arrives at Colhen's Inn.
Spoiler: The Real Oracle? Despite Pontiff Laurys's claims, Seanna is not the prophesied oracle. Tieve, another oracle of Morrighan, has both her body and her soul being transferred to Erinn, and in Episode 10, she becomes Morrighan herself. Additionally, the prophecy about Morrighan happened to be a lie made by the Pontiff's Court; Laurys was indeed working with Verafim and in order to cover his plans up, he announced Seanna was the prophesied oracle.