In 18 BBY, Darth Vader and three Inquisitors went to Mon Cala with over a dozen elite clones, named Purge Troopers, to investigate a possible Jedi infiltrating the monarchy there and advising King Lee-Char in his negotiations with the Galactic Empire. The Jedi was former Jedi Ferren Barr and he was hiding in a cave refuge near Bel City with six disciples. Barr intended to drive Mon Cala into war and soon became afraid that Lee-Char's negotiations may be successful and benefit the planet. Barr therefore sabotaged the Lambda-class T-4a shuttle carrying Ambassador Telvar and as Vader and his Inquisitors arrived at Dac City, the shuttle exploded, killing all on board.
The assassination prompted an Imperial invasion of Mon Cala and a large battle broke out in Dac City, where the King had been negotiating with Telvar. Vader and the Inquisitors quickly took the landing pad and fought their way to the palace where one of the Inquisitors, the Ninth Sister, interrogated Lee-Char until he gave her the location of Barr. The battle of Dac City ended with a school of large creatures causing a tsunami to rip through the city. Vader and the Inquisitors, however, survived, only losing two Purge troopers. The Ninth Sister told Vader the location of Barr and they began traveling towards his refuge. Lee-Char later awoke after being injured from the cataclysm and realized that Barr's location was in peril. Without hesitation he contacted Barr, telling him Vader was likely on his way.
Barr immediately ordered his disciples to begin the evacuation protocol. When Verla asked what was happening, Barr responded by saying Vader was on his way. Barr set his R1-series astromech droid Endee to self-destruct whilst the others dressed into scuba-suits. Once they were suited up, Barr and his disciples mounted on their aqua-speeders and fled the refuge before Endee blew it up. However they were too late as Vader and the Inquisitors arrived on their Imperial submarine.
One of the disciples shouted for them to scatter but Nipaltoo was already attacking the submarine. His shots made no effect on the submarine and Vader crushed his helmet, killing him. The others fled the scene and Barr told them that they would have a chance if they went to Bel City and they continued their course with Vader catching up. As Vader got on top of them, Orvek got his blaster out and Verla asked him what he was doing. Orvek told her he would slow them down and he too attacked the submarine before being reduced to ashes. The rest of them made it to Bel City with Vader still at their heels and when they surfaced at a docking bay, Rebb decided to stay behind and slow them down as well. Stell immediately pledged to stay with her and Barr took the other two to find a landing pad in the city.
Vader and the Inquisitors arrived at Bel City and marched through it reinforced by their twelve Purge troopers. Vader then received a message from Moff Wilhuff Tarkin, requesting him to capture Lee-Char and end the fighting on Mon Cala. Vader refused but Tarkin convinced him to by pledging to owe him a favor if he did so. Vader left the Inquisitors and they continued the hunt themselves. Barr and his last two disciples made it to a landing pad but found it was empty. Barr began to panic and was about to suggest they turned around when the Inquisitors arrived and the Sixth Brother cut Daren down with his lightsaber. The Inquisitors and their troopers then surrounded the two and Verla attempted to confront them, only to be stopped by Barr who had a better idea.
Barr stepped forward to the Inquisitors and dropped his lightsaber. Barr stated the Inquisitor's original names from the time when they were Jedi: the Tenth Brother being Prosset Dibs and the Sixth Brother and Ninth Sister being Bil Valen and Masana Tide. Barr claimed they were still Jedi and the Ninth Sister identified him as well Ferren Barr. Barr told them how he had studied the past and the role of the clones, using the force to remove the helmets from the Purge troopers. Barr identified that they had only been mobilized after the Great Jedi Purge and hadn't had their inhibitor chips activated yet. The Sixth Brother then pointed his lightsaber at Barr saying he was going to die anyway. But Barr told them that they would always be Jedi and used a Jedi Mind trick, telling the troopers to "execute Order 66".
The trooper's inhibitor chips responded and forced them to turn on the Inquisitors. The Tenth Brother attempted to protest to them but they open fired. The Inquisitors began deflecting shots at the troopers, killing three of them before the Tenth Brother was hit in the chest and killed. Barr and Verla then leaped over the battle, escaping on the other side. As they made down the hallway, Barr told Verla to get off Mon Cala and find another Jedi before parting with her. The Sixth Brother was keen to catch up with them and the Ninth Sister suggested that they push the troopers away. The two then force pushed them aside, making way for them to escape. However the Sixth Brother slashed the Ninth Sister's right knee, cutting her lower leg clean off and leaving her to fend off the remaining troopers.
Vader eventually reached Lee-Char's Royal Command Bunker and disarmed him after dispatching his five guards. Vader made Lee-Char watch as Moff Tarkin began bombarding Mon Cala and Barr soon arrived to face him. The two dueled in the bunker and Barr revealed he had assassinated Ambassador Telvar to start the war on Mon Cala. Lee-Char overheard this and immediately ordered a ceasefire of his people. Tarkin accepted his surrender but continued the bombardment of Mon Cala until Lee-Char had been officially arrested. Vader slew Barr and took Lee-Char off Mon Cala. At the same time, Verla had escape Bel City and was hiding away in a cave with two Quarren soldiers. The Sixth Brother had too left there and took a boat away with three stormtroopers. In Bel City, one Purge trooper remained at the battle scene and knelt over the severed head of one of his brothers.
- Star Wars: Timelines