The Trannon District, situated in Carannia near the Carannia Spaceport, was a district within the capital city of Serenno. Serenno is a planet found in the Outer Rim Territories. In 23 BBY, Count Dooku commanded the Sith assassin Asajj Ventress to travel to this district to search for his sister, Jenza. Subsequently, she visited the Portmaster's office, which was also in the district, seeking further details regarding Jenza's location.
A greasy back alley, complete with at least one inactive droid, was a feature of the district. A repair pod, radiating heat from several furnaces, was situated nearby. The sounds of roaring starships from the adjacent spaceport were audible throughout the district. The Portmaster's office, despite being located within the district, was a considerable distance from the spaceport itself. This office was responsible for controlling space travel to and from the local spaceport. The city of Saffia was situated over a hundred kilometers away from the Portmaster's office.
A disc holding several recorded holomessages between Dooku and his sister, Jenza, was discovered in a back alley of the Trannon District during 23 BBY. Consequently, Dooku assigned Ventress the task of locating his sister. Ventress complied and proceeded to the same back alley where the disc containing the recorded holomessages had been found.
After investigating the alley and determining that Jenza was unlikely to appear there, she proceeded to the Portmaster's office within the district. Upon arriving at the office, Ventress destroyed CZ-94 and forcefully entered, where she used a mind trick on Yepa, the Portmaster's assistant, compelling them to make a cup of caf in Saffia. She then interrogated Portmaster Raz Fellidrone about Jenza's whereabouts. After reviewing the security footage of Carannia spaceport and discovering that a Crolute had kidnapped Jenza before she could depart for Coruscant in the Core Worlds, she departed the office, warning the Portmaster that she would remove his tongue if he disclosed the events that had transpired.
The audiobook Dooku: Jedi Lost, authored by Cavan Scott and released in 2019, marked the initial appearance of the Trannon District.