River Song (Karla-Gard's universe)
River Song was an alternate version of Melody Pond who hailed from a parallel universe.
She assassinated her universe's version of the Doctor, before turning on her captors during the Massacre of Demon's Run. When she would later recount the incident, she would admit she didn't even know the Doctor's name. She acquired a TARDIS and began travelling the universe, living through multiple incarnations including one resembling the River Song of the Doctor's universe, but soon found she was being pursued by the Karla-Gard.
She accidentally slipped through into the Doctor's universe via a rip in dimensions where she encountered an alternate version of herself. By that point she had gone through a few of her regenerations. Together they defeated the Karla-Gard and she successfully returned to her own universe, where she sought out her own world's Professor Dern. (AUDIO: The Two Rivers)
