First off, you should be aware this: the Ancient Nightmare can only be found on Wednesdays and on Fridays at 12pm or 8:30 pm. Then it's at 10 pm (local server times). If you're looking for the boss in any other hour D2R Items, you'll be sorely disappointed by the things you don't discover.
If you're playing Diablo Immortal on the correct time and date however, you should head for your Misty Valley area of Mount Zavain. The Ancient Nightmare patrols the area surrounding the Misty Valley Waypoint. If you're within the general region, you'll receive an email indicating that the boss is summoned and that you'll be able auto-navigate to the area. The boss is massive, Xenomorph like creature that is surrounded by a glowing shield. It's kind of hard to miss.
The fight with the Ancient Nightmare is a little tricky. It requires you to remove the shield of the Ancient Nightmare so it can begin harming him. Although the Ancient Nightmare's attack seem to be fairly simple but it's going take a while to bring that shield down and begin to eat away at its health. It's easy to conclude that everything is impossible.
Despite plenty of controversies, Diablo Immortal is determined to prove that it's not just an outlet for your spare cash. Today's update, titled Forgotten Nightmares, is the most significant since cheap D2R Ladder Items the game's launch which brings players a new dungeon, a brand-new set of items as well as an expansion for your Warband Castle.