After the events of Star Wars: Revenge of the Sith, Darth Vader is thrust into his Sith training. His first step is to obtain a crimson lightsaber, the signature weapon of the Sith. To craft his weapon, Vader has to defeat a Jedi in battle, take his lightsaber and torture the Kyber crystal inside with his pain and anger, a process which makes the crystal bleed and "sing a hymn of darkness," according to Emperor Palpatine. Tracking down a Jedi after Order 66 is tricky but Anakin's knowledge of the Jedi Order leads him to Kirak Infil'a, a Jedi who has been living on the river moon of Al'doleem, in 2017's Darth Vader #3-4.
During his time as a Jedi, Kirak's main purpose is to be a warrior. His green lightsaber shows his abilities as a great meditator and negotiator for the Force. He's known as a master in lightsaber combat, but his time in isolation is spent meditating on the Force and he greatly strengthens his abilities. Before Vader's arrival, Kirak is seen completing some incredibly high-level training that involves assembling puzzles with the Force while simultaneously lightsaber-dueling with his robotic assistant.
Kirak takes the Barash Vow at some point during his time as a Jedi Master, as penance for crimes he committed as a Jedi. Kirak declares the vow is completed after his first encounter with Vader. Kirak senses all of the hate and fury leftover from Order 66 in Vader and Vader's incredible connection with the dark side forces Kirak out of retirement so he can restore the Jedi order.
Kirak's first duel with Vader not only shows the true extent of his power, but his cunning as well. He forces Vader to evade a litany of booby traps on his way up the mountain, where he challenges Vader to a duel. First, he throws an entire river at Vader and then a flock of giant flying raptors. After Vader survives, Kirak decides to turn off the rest of the traps and allow Vader safe passage up the mountain. Vader kills Kirak's dueling droid and attacks Kirak with the training saber.
Master Infil'a quickly takes advantage of Vader's weakened state. After just a few strikes with his blade, he has Vader on his knees with his green blade at Vader's throat. He then Force pushes Vader off a cliff, letting him fall to his presumable death. However, Vader doesn't die. He uses the training droid's remains to repair his robotic limbs enough to continue his pursuit of Master Infil'a's lightsaber.
Kirak quickly makes his way to Am'balaar City, which is built on the dam of a giant river, to recover his starship so he can pursue Vader's master. While he waits for his starship to be repaired, Vader tracks him down and attacks him. Kirak and Vader duel on top of the dam and Kirak is on the verge of gaining the upper hand once again. However, Am'balaar City Police intervene to stop the duel and protect the dam. Vader takes advantage of their distraction and throws them off the edge.
Kirak quickly turns his attention to the falling police and saves them. "You are a monster," he declares. This is the moment Vader realizes he has the advantage. Vader immediately responds, "Yes. I am." He attacks to the dam with Force. Master Infil'a turns all of his attention to hold the water back from the city. Vader puts his training saber away, finds a crack in Kirak's Force shield, and holds him high above the water with his signature Force choke ability.
Vader forces Kirak to watch the city flood and wash away, then snaps his neck and throws his body into the water with an apathetic gaze. Although Vader ultimately defeats Master Infil'a, it isn't because he overwhelms him with the Force. Vader has to prey on his Kirak Infil'a's willingness to put human life above his own self-interest to defeat him, because the Jedi is too powerful to defeat in regular combat.