Venturing through the underground catacombs, Nate made his way into the monastery chapel, where he found himself in a big shootout with Atoq Navarro's mercenaries. Soon after, he spotted two large bells, which he recognized from Francis Drake's diary. He turned to a page that read, "Two bells resound in perfect harmony." By getting the bells to chime at the same time, he unlocked a secret passage that led him back underground.
As Nate traveled back underground, he soon reached a room beneath a small mausoleum, where he could overhear a conflict occurring between Eddy and Roman. Eddy tried explaining to them that the island was cursed, as his men were being massacred by something other than Drake. Due to Raja and his crew failing to capture Drake and keeping the island secure, Roman dismissed him and terminated his contract, therefore Eddy would not receive a share of the treasure as promised. Enraged, Eddy pulled his gun on Roman, only to be stopped by Navarro. With no other choice, Eddy left the room, cursing in anger.
Roman then began questioning Navarro on his ability to find the treasure. Navarro was positive the treasure vault was in this monastery but claimed he needed a little more time. Roman believed this treasure hunt has proven to be more trouble than it was probably worth. Navarro assured him that El Dorado was worth more than he could imagine, which Roman was counting on. After overhearing their conversation, Nate continued through the catacombs and was soon led back outside.
After following the stairs down into the first chamber, climb up the ledge to your right to enter into the next area, where you will need to hang off the ledge on the right and shimmy your way across to the other side of the gap. After ascending the small stairs, prepare to be ambushed by a group of mercenaries, who will immediately begin firing at you upon detection. They are, however, not difficult targets if you take cover behind the crates. Ideally, any enemies wielding the Desert-5 should be your first target. When this group is defeated, search for a chain winch along the back wall. Activating it will raise the gate blocking the large flight of stairs that will take you up into the large church seen earlier.
Quickly take cover behind the first pew, as you will find the whole room is guarded by patrolling mercenaries, with a few of them wielding Desert-5's at the back. The pews will provide sufficient cover, and you can also find a few Mk-NDI grenades lying around. There will not be that many enemies in total as well, so once you have shot everyone down, approach one of the two large bells and pull out Drake's diary. Nate will turn to a page that displays two bells, hinting that you need to have the two bells ring simultaneously. This can be achieved in two ways. You can approach a bell and press to ring it, then quickly shoot the other. Your other option is to simply shoot both bells quickly.
When done correctly, a secret door on the far left end will open up. Pass through to have Nate climb down a ladder that leads back underground. Once at the bottom, prepare to shoot down more mercenaries who you will run into by surprise. Having a Desert-5 or SAS-12 can be useful when fighting these enemies in such tight spaces. Continue following the path, which involves more platforming sequences and a cutscene. You will eventually reach a room with a winch that you can activate to open the door to the outside.