Dungeon Ender Achievement
Complete every Dungeon Door in the Overworld.There are actually only seven doors in the Overworld that count for this achievement. All that counts are the metal doors that hold shrine pieces behind them. They are marked on your Overworld map with a green leaf-like symbol (and if you put your cursor over them, it'll say shrine piece). If you've already completed a door, it'll have a white banner next to it on a post in the Overworld, and it'll be checked off on your map.
You won't be able to reach the final doors until you progress far enough in the story that you finish Karnak's Wall. Once you reach the upper portion of the Overworld, you'll have access to all seven doors. Each door consists of three combat encounters. The first two are your standard ones where you have to kill enough enemies to fill the meter on the right to complete the encounter. The third encounter has an obelisk in the middle, and you'll need to kill enemies to charge it up. Eventually, a miniboss will appear, and killing it immediately completes the encounter, so focus on it and ignore the regular enemies. Once dead, a shrine piece will be your reward.
Each door you complete is added to the in-game challenge tracker for completing all seven dungeon doors in the Overworld.