Cubes

Legendary cubes can be obtain killing certain bosses or completing certain dungeons/instances, some can even be obtain with multiples ways. Bellow a list with all cubes in the game.

They can be opened one time for free and two more times paying the correct currency.

About
Each of them has a loot table and what you get is random. They contains gear pieces, soul fragments or material for crafting. A good tip is to always open them 3 times, increasing your chances for a certain drop you're lucking for.