Far Cry 3 This Knife’s For You follows Unhappy Reunion in the main story quest. After killing Buck you managed to keep the ancient knife in your own property.
It is time to give it back to its owners.
Exit the cave.
Bring the knife to Citra.
Citra can be found at Citra’s Temple. In order to get there fast travel to the Temple Gateway position. Upon reaching the big tree at the top of the stairs you will meet Dennis. Soon after that Citra will join and you will give her the ancient knife. She will take you to the nearby room and, after admitting her that your new desire is to kill Vaas and Hoyt, she will talk about the isle history and the history of their people. She promises that the power you want in order to defeat Hoyt and Vass will be given to you if you help them release their captive warriors.
Leave Citra’s temple.
Leaving Citra’s temple will make you meet Dennis again. He will inform you about the ambush they are preparing against Vaas’s convoy that is carrying the prisoners. They will need your help for the ambush to succeed.