Lyta told the station crew that she knew there was a Psi Corps spy on B5, but she didn't know who it was. The spy was in the form of a telepathically implanted personality; the person would not even realize he/she was a spy. The entire crew undergoes questioning, with Lyta present trying to trigger the false personality; it doesn't work. Finally, Talia walks in to talk to Sheridan; Lyta sees her and on a whim tries to trigger the personality. To everyone's shock, it works! Talia becomes Dark Talia, totally loyal to Psi Corps. Sheridan locks her up and demands Psi Corps remove her from the station, which they do. Later, when Bester visits the station during Season 3, he mentions to Garibaldi that Talia had been killed and dissected to try to determine the cause of the increased mind powers she received from Ironheart during the episode "Mind War".
End of spoiler!