There are 100 invisble treasure boxes hidden around the world map. You can open them without knowing, but once you get Tengri, you will get the spell Seek Fortune which will show them on the continent/island you're currently on, and cause an exclamation mark to appear over your head when you are near one. As soon as you get Tengri, you can get 95 of them, but you won't have access to the last 5 until near the end of the game. Here's a map of the 100 chests (the red markers) - World Map (IGN).
One thing to note. The boxes around Yule (the ice town) tend to be surrounded by little cliffs and such. You may try to land Tengri near one and get stuck and be unable to call Tengri to you, so you'll have to teleport out.