In brief: It's almost always the case that if you want to play some DLC, you need to own the base game that goes with it—but not with Doom Eternal's first single-player expansion, The Ancient Gods, Part One.
A trailer for The Ancient Gods, Part One arrived during Gamescom's Opening Night Live, and it looks very exciting, with new locations to explore and new demons to annihilate. The clip also shows a player taking on two Marauders, probably the most annoyingly difficult enemy in the game, simultaneously.
Despite being an expansion, The Ancient Gods doesn't require the base game. "When we look at how many people have played through the game we have a large percentage – higher than 2016 – that have completed it, so you're always looking at that and seeing how far players make it through your game," id Software executive producer Marty Stratton told PCGamesN. "It’s important as we want the DLC to reach as many people as possible. In fact, you don’t even need to own Doom Eternal to buy the DLC and play it.”
Another difference from usual pieces of DLC is that Ancient Gods will be pretty large. "Most people would associate DLC with being of smaller scope – it's kind of like the made for TV version of the movie, and the movie was the main game," said Game director Hugo Martin. "For us, this is very much like a two-part film – this is every bit as grand as the main game was."
The Ancient Gods, Part One releases on 20th October, 2020. As for Part Two, there’s no official launch date, but Stratton said it would arrive within one year of its predecessor.