Thanks a lot to LeMagnesium, crabman77, Ombridride, Gael-de-Sailly, ObaniGemini and other people from server-minetestforfun for working on this mod.
After installing this mod, you may want to configure the nether.trap_players setting to choose between an adventurous nether or a less stringent nether.
After you've entered the game, just make a portal like the one of PilzAdam if you enter the nether the first time.
When you arrive, type "/nether_help" to get information about tools, recipes and much more.
lkjoel's version: viewtopic.php?p=21486
Information about dependencies, license and download can be found at https://codeberg.org/HybridDog/nether-pack.

Spoiler

Thanks to sss and WSDguy2014 for reporting bugs and ideas (e.g. the nether forest).
textures edited with gimp
If you got ideas or found bugs, please tell them to me.
How to install a mod?