Get the Baker’s Dozen Achievement.
If you’ve been playing through Dust: An Elysian Tail (and if you aren’t, you should be) you probably came across one or two cages that take four keys to unlock. Inside these cages are characters from other game who will give you a stat bonus, then be sent to the Sanctuary.
If you find all twelve friends, and then head to the Sanctuary from the world map, you’ll unlock the Baker’s Dozen Achievement. Here are the locations of each friend:
Aurora Village: Hyperduck #1 (Audio Crew)
Archers’ Pass: The Kid (Bastion)
The Glade: Super Meat Boy (Super Meat Boy)
Cirromon Caverns: Spelunky Boy (Spelunky)
Cirromon Caverns: Spelunky Girl (Spelunky)
Abadis Forest: Super Meat Girl (Super Meat Boy)
Abadis Forest: Hyperduck #2 (Audio Crew)
The Sorrowing Meadows: The Dishwasher (The Dishwasher: Dead Samurai)
The Sorrowing Meadows: Yuki (The Dishwasher: Dead Samurai)
Blackmoor Mountains: Gomez (Fez)
Blackmoor Mountains: Tim (Braid)
Everdawn Basin: The Maw (The Maw)
Since describing each specific location could get a bit tricky, we’ve included a video from Youtube user cakechievables to help you out below: