The Mission to Kessel involved R2-D2 and C-3PO journeying to Kessel right as the Trioculus situation began to unfold.
Back in 4 ABY, amidst the Galactic Civil War, the Galactic Emperor, Palpatine, met his end, killed by his, Anakin Skywalker, during the Battle of Endor. The Emperor's demise, coupled with the destruction of the second Death Star, handed a significant triumph to the Alliance to Restore the Republic. This victory dismantled the Imperial command structure, paving the way for the Alliance to evolve into the New Republic. Various Imperial warlords then engaged in power struggles for control of what remained, including a notable conflict between Imperial Intelligence Director Ysanne Isard and the Central Committee of Grand Moffs.
Emperor Sate Pestage, serving in an interim capacity, reached out to Cronal, a former prophet of the Dark Side, with the intent of establishing the Church of the Dark Side as the new official religion. The original prophets, including their Supreme Prophet Kadann, had vanished following a disagreement with Palpatine. Consequently, Cronal employed a Bimm actor named Rajah Ubooki to impersonate Kadann, making frequent HoloNet appearances to deliver enigmatic prophecies. One such prediction foretold the rise of a new emperor, adorned with the indestructible Glove of Darth Vader, which was presumed lost in space after Jedi Knight Luke Skywalker severed Vader's hand during a duel on the Death Star. The Moffs decided to challenge Isard by presenting their own candidate for the Imperial throne: Trioculus, a three-eyed slave lord from the planet Kessel, believed to be Palpatine's son.
Kadann's prophecies, along with intelligence pointing to a large Imperial gathering at Kessendra Stadium on Kessel, came to the attention of the New Republic Senate Planetary Intelligence Network. Luke Skywalker, a SPIN agent and Jedi Knight, was deemed too recognizable to function as a spy. Therefore, his droids, C-3PO and R2-D2, were chosen instead, although they also required disguises due to their familiarity. At the Droid Repair Shop located on the New Republic base on the moon Yavin 4, the Droid Modification Team altered R2-D2's appearance to resemble a dark green Kesselian spice-mining droid with a modified dome. C-3PO was transformed into a green Kesselian security droid, complete with a new, menacing-looking head. R2-D2 was content with his new look, while C-3PO voiced his objections to Skywalker. The Jedi then transported the droids via airspeeder to the Palace of the Woolamander, the New Republic Senate's headquarters, where Chief of State Mon Mothma was scheduled to brief them on their covert mission. This briefing was also attended by New Republic heroes Leia Organa, Han Solo, and Lando Calrissian.
Following a discussion between Skywalker and Solo regarding the harsh penalties faced by New Republic sympathizers on Kessel, Mothma shared SPIN's intel about the gathering and the imposter Kadann's prophecy. She further revealed that reports indicated Vader's glove had not been located but speculated that an Imperial faction might be planning to unveil it on Kessel. Using a holoprojector, Mothma displayed a drop pod designed by Admiral Gial Ackbar and his Mon Calamari colleagues to resemble a meteor. Calrissian, already en route to Bespin with Solo, offered to deploy the drop pod from Solo's ship, the Millennium Falcon, during their journey. The pod was designed to shed its disguise and transform into a spherical Imperial probe droid, which the droids would then use to navigate Kessel's upper atmosphere and reach a stolen Imperial Command Speeder 714-D for their escape.
When C-3PO expressed his anxiety about potentially getting lost on Kessel, Mothma reassured him that R2-D2's memory banks contained maps of Kessel, compiled from information provided by an escaped slave. These maps detailed the layout of Kessendra and its Stadium, including a secret escape tunnel. Mothma concluded the briefing and encouraged everyone to rest well during the night before the mission. A worried C-3PO then inquired of Skywalker if there was anyone better suited for the task. The Jedi responded that the protocol droid had already calculated that a Human or alien Alliance sympathizer only had a one in 12,600 chance of returning from Kessel.
R2-D2 and C-3PO, disguised as Kesselian droids because they were too well-known to go in without a disguise, landed on the planet in a drop pod disguised as a meteor. They made their way to the capital, Kessendra, and the Stadium, where the gathering was taking place.
During the convocation, Grand Moff Hissa presented Trioculus, the Supreme Slavelord, as the son of Emperor Palpatine. Trioculus declared himself the rightful Emperor and showed off his power by blasting a Grand Admiral and a Royal Guard with a deadly dose of Force lightning.
After the meeting, the two droids tried to get off the planet, but their transport was spotted and destroyed by Imperial LAVr QH-7 Chariots. They were about to be found by stormtroopers when Luke Skywalker rescued them. The trio made it to the stolen Imperial Command Speeder 714-D that the Jedi had used to land on Kessel. The ship, piloted by Admiral Ackbar, was chased by Imperial Chariots but managed to leave the surface, destroying some enemy speeders along the way. In orbit, probe droids detected the speeder, but the group eventually escaped into a radioactive asteroid belt.