Molten, volcanic worlds with oceans of lava and heat far beyond what baseline humans can endure.
Expect real temperatures of around 70-110°C planetwide. Nearly nothing can grow or live on the surfaces of these planets, and attempting to establish a colony on one unprepared is nothing short of a suicide mission.
This mod is a planet type for the Alien Worlds Framework, check its workshop page for FAQs.
Worldbuilder-compatible, but also works standalone.
Source code: https://codeberg.org/maevi/rimworld-mods/src/branch/main/hellworld