The 62-year-old government officer from Swansea joined existing Traitors Rachel and Stephen in the turret, expressing her delight at being “drunk on power”.
This series first saw host Claudia Winkleman select a hidden Traitor whose identity was kept secret from everyone, including the audience, until this evening's reveal.
Fiona will now operate as a regular Traitor alongside Rachel and Stephen, though initial interactions hinted at a potential lack of complete trust among the trio.
In another dramatic twist, Claudia Winkleman revealed to the group that a banishment would happen before the challenge in the episode.