This is a paid asset, but now you can download Forbidden Dungeons Free.
Detail this asset from Unity Store: Original Link
Forbidden Dungeons v1.1
“If possible, please buy the package to support the developer”
With this pack you get the tools needed to create a dungeon environment. It can be customizable to create your own underground level. You can also mix the pieces with other environment tools. Included many lods in heavy models to improve performance. Inside this asset: 206 prefabs with different blocks to build scenes, decoration props, fxs, shaders, materials, high quality textures, etc.
Right now it includes 2 main zones (big room & sewers) wich also includes corridors, extra rooms and 2 different sewer style (small & big). This underground pack is constantly growing with every update including new different zones & assets.