Serenno shows up in the locations list on the
OS episode guide.
Episode Guide: Nightsisters
"The swiftest path to destruction is through vengeance."
Synopsis: Darth Sidious, troubled by Ventress's growing prowess with the dark side of the Force, commands Count Dooku to eliminate her. When Ventress survives Dooku's assassination attempt, the jilted former apprentice vows to take revenge, enlisting her kinswomen -- the Nightsisters of Dathomir -- in her scheme.
Director: Giancarlo Volpe
Writer: Katie Lucas
Key Characters: Asajj Ventress, Count Dooku, Mother Talzin, Karis, Naa'leth, Obi-Wan Kenobi, Anakin Skywalker, Darth Sidious, R2-D2
Key Locales: Dathomir, Serenno
It's an EU world that's Count Dooku's home planet (where he gets his title from) and probably where he gets attacked in his sleep in the earlier preview clips.
