You can get this in the town of Kuwaq Yaku, you'll get here as part of the game, so you can't miss it.
The mission is called Deal with the Pillagers.
When in town, wander around pressing
until you see a guy light up green (there're a few that light up green) with what looks like an Aztec head over his head. This is the guy who will give you the mission, his name is Omar, and he's roughly in the middle of town past the bar.
From this point follow the green markers on the map, you will:
Talk to Omar
Talk to Marco
Head to the edge of town
Enter the crypt worksite
Defeat the bad guys
Return to Marco
You will also get:
Assuming this is your first side mission!