While Lord Maranos may drop a variety of D2R Items random rewards, what you're most interested in is his guarantee of drop the Zakarum Sigil. What makes this drop so crucial? It's because they aid in making the tough fight with the Ancient Nightmare just a touch more manageable.
The first thing to do is be aware you can only find the Ancient Nightmare can only be located on Wednesdays and Fridays at 12pm at 8:30 pm, 12 pm, and at 10 pm (local servers time). If you're looking for the boss at other times you'll be terribly disappointed with what you can't discover.
When you're in Diablo Immortal on the correct time and date however, you should head towards in the Misty Valley area of Mount Zavain. The Ancient Nightmare is a threat to the region around and around the Misty Valley Waypoint. If you're in the region, you'll receive an alert that indicates it's been summoned by the boss, and that you can auto-navigate to the area. In addition, the boss is an enormous, Xenomorph-like creature which is protected Diablo 2 Resurrected buy items by an illuminated shield. It's hard to ignore.