If you need a way to quickly populate your game with enemies and NPCs, Spawn could be the ticket. This AI tool is geared for D&D enthusiasts, able to generate fully realized characters, campaigns and enemies in less than a minute. It'll spit out attributes like hit points, armor class and abilities, and can be a huge time saver for game masters.
Another good choice is LitRPG Adventures. The site employs large AI tools like OpenAI's GPT-3.5 and DALLE3 to create a lot of game material, including monsters, quests and NPCs. It's got a big library of more than 100,000 entries and offers several tiers of membership plans for different budgets. It's a good general-purpose tool that can handle a variety of campaign settings, from traditional fantasy to sci-fi future.
If you need something more general purpose for creating assets, check out AI Game Assets Generator. Although it's geared for game developers, it can create a lot of different game assets with natural language prompts. It can generate characters, weapons, scenes and more, giving you a lot of options to flesh out your campaign.