Collect 150 faction items.

Each of the 3 factions have special items to be collected as you progress through the game. As you complete more missions for each of the factions more items will be revealed on your map. Open your PDA and look for the small blue icons that seem to glow a little bit, these are the faction items. See below for more info:

The Reapers 100 Black Boxes Of Contraband. These seem to be located more near the waters edge of the island, some are even in the middle of the ocean.

The Roaches 100 Drug Drops In Suitcases. These are located all around Panau, specifically in the towns.

The Ular Boys 100 Ancestral Skulls. These are mostly located in isolated areas, such as the big deserts or high up in the mountains.

When you get close to an item a grey arrow like the one for the resource items will appear on screen to point you in the right direction. You can also spot these just like the resource items, from far away or high up look for a small shine because it could be one of these. Just collect 150/300, it doesnt matter how many you get for each faction just as long as you total 150.