For The King

Can only be unlocked during the Dungeon Crawler Campaign.
In every game of Dungeon Crawler there are 5 main dungeons out of 7 overall.

Every biotop (desert, grass, bad land,...) has only one dungeon. Meaning, if you cleared one in the desert, there won't be a second one.

The main dungeons can be found in the following areas
1. Glittering mine (grass)
2. The obsidion keep (bad land / vulcan land)
3. Flooded crypt (poison swamp)
4. Water temple (isle)
5. Buried temple (desert)
6. KIng's Maze (golden plain)
7. Mage's Dungeon (Burning Forest)

After I found 2 out of the 3 main dungeons, which gave me an achievement in my first run, I started a new Dungeon Crawler Campaign, bought 2 Vision Scrolls and tried my luck to find the area I was searching for. After two more restarts, I found the right area and the missing main dungeon.

During the Dungeon Crawl campaign, “the obsidian keep” can be one of the five “realm dungeons” of the main story objective. The location, level and order is likely randomized. This may not always appear either every campaign (thanks eohjay for your comment). The boss battle is against an ember demon (has physical and magic armor and immune to fire) plus two bats (evasive so you need perfect rolls to do damage). There’s two levels in the dungeon - not all are battles. Beat the boss then pass a Talent roll to destroy the chaos generator and the achievement will pop.