You'll find loot chests with two 20 sided dice on top in various places on the maps. You can roll one or two die (depending on if you want to spend eridium or not) to try and get better loot. The higher the roll, the higher the quality/rarity of the loot spawned inside. The highest die is used to determine quality of loot. The first dice chest can be found in the northeast part of the Unassuming Docks near the broken temple looking area (The Forlorn Ruins) down on the waters edge. The highest die must be either a 1 or a 20 to pop the achievement. If you don't pop the achievement by the end of the main story, once you defeat the Raid Boss (see Make it Raaaaaid for more info) you will gain access to another 53 of these chests right there.
Easter Egg for 3 more chests: After defeating the raid bosses when you're going through opening all the other 53 chests, there is a balcony (furthest south center of map) with 3 breaks in the chain railing and a lot of rakk looking creatures flying in the background. If you stand on the balcony watching them long enough a short but epic battle will be waged in the background and then 3 more chests will rise out of nothing filling the blank spaces in the chain railing. Probably more worth it for the epic battle than the loot chests. (thanks to DustyTheHampster for the heads up on this)