Far Cry 4

In order to unlock this achievement, you must first progress through the campaign until you see a green 'Y' appear on your map. This will take you to the Ghale Homestead and you will meet Yogi and Reggie.

This first encounter with the duo leads into a mission involving the Arena which ties in with the main campaign and must be completed in order to unlock the next part.

Your second encounter (they will still be located at the Ghale Homestead) will have Yogi and Reggie start mentioning Shangri-La and they will tell you of a 'door with a really scary face on it'. Go to the waypoint highlighted on your map, a location called 'Kalinag's Wisdom'. Be careful as there a handful of hunters (they can charm animals to fight against you) outside the entrance and a couple inside as well.

Proceed down and through the cave using your grapple hook where necessary and you will find a piece of a Thangka on a table which acts as a portal into Shangri-La.

Once you are here you will have to make your way through (there is really only one main path so getting lost isn't an issue and you will eventually get waypoints to specific items you need to interact with) to activate the Bell of Enlightenment. There are groups of demons you will have to slay along the way and your only weapon is a knife. They do not put up much of a fight and are easily dispatched with takedowns; if you have unlocked the Knife Throw Takedown/Chain Takedown skills on the Tiger tree this will make killing them all the more easier.

After activating the Bell of Enlightenment you are returned to Kyrat and view another cutscene with Yogi and Reggie. After this finishes you are awarded the Overdose achievement.

EDIT: After completion of the above you will now be able to improve the Ghale Homestead by purchasing items from the caretaker outside the house. Buying any three of these will also earn you this achievement: