This was taken from a post on reddit, don't have the link http://youtu.be/lo0RcoRc1l0 At around 13:00 Destin begins to talk about the keys. He states that they're used for the end chest after you beat the prison of elders. Basically, let's say you beat the final boss on Level 34 PoE, he drops a Treasure Key and after, you go into a room with a big chest. You open the chest using the key that just dropped for you and you get you're rewards (Legendaries, Exotics, Etheric Light etc). Now let's say you run Level 34 PoE again, when you beat the boss a 2nd time he won't drop a Treasure key this time, however if you have one in your inventory, you can use that key to open the treasure chest at the end when you get to it. In case you're wondering, you can only use one key per run. You can't use all 5 keys at the same chest So yeah, farm the hell out of those keys. You'll need them.