After finding the Scythe, Buffy traveled to the Guardian's Crypt to find answers about the weapon. There, Buffy encountered the last remaning Guardian who filled her in on the history of the Scythe. After explaning its purpose, the Guardian's neck was snapped by Caleb who taunted Buffy before engaging her in a fight.
The First Round
Buffy and Caleb engaged in a fight to the death against each other, but due to having merging with The First recently, Caleb was more powerful than ever and Buffy had to struggle just to keep up. Though landing a few blows, Buffy proved to be no match for Caleb. She was knocked down with Caleb ready to kill her with the Scythe when Angel suddenly joined the fight. Angel knocked Caleb out for a moment, saving Buffy from death and allowing her a moment to recover her strength.
The Second Round
Caleb recovered fast and Angel tried to help Buffy, but Buffy had Angel just watch as she wanted to fight Caleb alone. Caleb and Buffy fought each other again and this time they were more evenly matched, even though Caleb was still the stronger of the two. Finally Buffy managed to duck one of Caleb's blows and sliced him across the stomach, apparently killing him with Angel, The First and Spike watching.
The Third Round
After everything seemed to be over, Caleb rose again and knocked Angel out, his incredible power from his recent merge with the First allowing him to survive his wound for at least a time. Caleb engaged Buffy again one on one, and while he was still incredibly strong and powerful, his wound seemed to have weakened him a bit, giving Buffy more of a chance. Finally, Buffy managed to knock Caleb to the ground and when he got up again, she swung the Scythe up into his crotch while he was taunting her. Buffy pulled the Scythe all the way up through Caleb, cutting him in half and finally killing him.
After Caleb's death, Angel gave Buffy an amulet and file on the First he got from Wolfram and Hart before returning to Los Angeles at Buffy's insistence. Buffy ended up giving Spike the amulet, and the First appeared to Buffy that night in the form of Caleb to taunt her. The First confirmed Buffy did in fact kill Caleb in the fight and tried to demoralize her. However, the First's taunting instead gave Buffy the plan to turn all of the Potential Slayers into Slayers to battle the First and its army.