Fallen Knight
Above Dagobah's surface
Cas had been assigned to track down the deserter Castor Azari after rumours surfaced of him joining the Sith with the goal of bringing him in for questioning. After their first and only mission together when he was a Padawan, Cas had always felt something was off with the former Jedi Knight and wasn't surprised at all when he found out that he'd eventually left the Concord... although, nothing could have prepared him for the fact that he had joined the Sith. Cas took this mission personally as he had seen signs of the former Knight's Dark tendencies and could have, potentially, put a stop to all of it. 
After weeks of tracking him down, Cas learned that Castor had indeed joined the Sith which angered him further. Cas and BB-5 had managed to track Azari into the Unknown Regions before eventually chasing him into the Dagobah system.
Things came to a head as the Jedi Knight opened fire on the Castor, thus engaging in a dogfight with him. Activating his deflector shielding, he prepared to open fire once more with the starfighter's laser cannons. But before doing so, he quickly addressed his astromech through the comms "BB, patch me through." he ordered with a hint of aggression in his voice, directed at Castor. Once BB-5 did so, Cas immediately spoke through to the former Jedi Knight "You damned traitor!" he yelled in anger before sending off to laser shots at him "I'm gonna make you answer for everything!" he concluded, preparing to shoot him down to the surface below.
Castor Azari