Far Cry 3 Lin Cong I Presume takes place after The Motherload main story quest. After acquiring the third and final part of ancient compass you are ready to find the ancient knife which will grant your friend Keith Riley freedom.
Go talk to Buck.
Fast travel to Amanaki Outpost and head north. Buck can be found near the south of the lake north of the outpost. After his history lesson you are ready to acquire the knife.
Dive into the cenote.
There is a rock at south side of the lake that is nearest to your position you can jump safely from. Leap into the lake.
Find the entrance.
After you land safely in the lake, you will need to swim into an underwater cavern and climb onto the ledge. From there you will need to find and climb the vines up to a path. This path will take you to a hidden entrance. Open the door and head inside.
Find the way to the tomb.
Finding the way to the tomb will consist of passing by empty tunnels, climbing up vines, and killing pirates and their dogs. There is one heavy gunner here so use caution as you make your way forward.
Follow the cable to find another path.
After clearing the pirates out of the first cave section you will need to head down to next part of cave using the zipline. Make sure to reload your weapons and to have full health pool before climbing down since you will encounter many pirates at the bottom. After you land pirates will be found scattered all over the cave. Take your time killing them and don’t go out on a open if you are not sure your sides are covered or you may find yourself being attacked from multiple sides. After using another zipline you will find yourself at a part of the cave where poisonous gasses are pouring out of the hot water.
Cross the hot water.
Touching any water in the area will be bad for you. Avoid stepping in the water by jumping on a rocks, making sure to wait for a geyser to stop erupting if is blocking your path before attempting to jump over it. Proceed carefully and you will find yourself walking into two big cave rooms filled with pirates. They will notice you pretty easily if you are not careful so try and take them down from a distance before proceeding. At the end of the second large cave room, after climbing up some vines, you will need to use your compass in order to open a rock door to next part.
Reach the tomb entrance, the knife must be inside.
Soon after you pass the rock door you will find yourself in a whole new cave area. A bridge to the center is broken so you need to lower yourself down and pass more hot water. When you reach the ground level go south, that will lead you over to a rock plateau in half circle and a vine you can climb on. After you climb up keep going upstairs until you reach the statue.
Find the real tomb entrance.
At the bottom of the statue there is a switch, allowing you to use the compass in order to move the statue away. This opens the hidden entrance bellow.
Open the sarcophagus and pick up the knife.
While progressing downstairs you will meet couple of komodo dragons. At the very bottom level you will find the sarcophagus that holds the Ancient Knife. Interact with sarcophagus in order to open it and take the Ancient Knife.
Avoid the traps, get out.
Taking away the Ancient knife causes the room to start collapsing. Sprint to other side of the room while passing by the fires that are going out of lion statues on a wall. At the end of the room there will be a small cave exit you need to pass as soon as you can. Press the correct buttons and you will soon find yourself out of the cave with the knife in hand.