Death World

Not to be confused with Deathworld, a homebrew alternate universe setting inspired by a picture of Barack Obama looking badass in a Soviet uniform.

A Death World (Mundus in Articulo Mortis, which is Latin High Gothic for "world at the moment of demise") is a classification of world used by the Imperium of Man to describe various worlds where everything on the planet is trying to kill its human population, much like the good ol' Ice Age. You know the old saying if you can't adapt to an environment, make it adapt to you? Doesn't apply on Death Worlds. You have predators the size of Land Raiders, environments so extremely hot or cold most don't make it into their teens (let alone out of them) and resources are so scarce that every human settlement has adopted a philosophy of "kill or be killed" toward outsiders. However notably the planet still has to technically be colonizable to be a death world. A gas giant may have 400 mile per hour winds and rains acid, but you can't colonize a gas giant. Hence death worlds only apply to worlds hostile enough to mean you would have to be crazy to live there, but you could live there if you were crazy enough to do it.

One must ask why the Imperium allows such worlds to exist, and there are two reasons. One: some of these worlds were colonize many ages ago and only became death worlds later on. Kreig which was nuked long after it had been settled and Chemos (Before it was properly exterminatused) are examples. In these cases the Imperium did not knowingly settle a death trap, but got one either due to enemy action or them losing dark age technology that kept it from not being a death world. Evacuating a planet is neigh on high impossible so the Imperium just tells the people to deal with the new problems. The other reason is that if you can survive all of that, then you're badass enough to become an Aspirant into the Space Marines. The leftovers? They become the most badass of the Imperial Guard.

Noted Death Worlds

• Baal and its moons, homeworld of the Blood Angels.
• Barbarus, homeworld of the Death Guard (subjected to Exterminatus during the Great Scouring)
• Caliban, homeworld of the Dark Angels (destroyed). Though they were determined to civilize it, consequences be damned.
• Catachan, homeworld of the Catachan Jungle Fighters, the Death World of all Death Worlds.
• Fenris, homeworld of the Space Wolves, both before and after Magnus came along and wrecked everyone's shit.
• Medusa, homeworld of the Iron Hands
• Nocturne, homeworld of the Salamanders.
• Cretacia, homeworld of the Flesh Tearers, AKA a planet with MUTHERFUKIN DINOSAURS!
• Krieg, homeworld of the Death Korps of Krieg, nuked by loyalists so Emprah could rule another bare rock. Shit version of Fallout
• Phyrr of the Calixis Sector. Yeah the air and waters are clean, and the whole place looks normal, but the spores and animals are lethal there as they have have some bizarre gene-toxin that kills grown-up people in an instant. So deadly that there is no cure for it, as the Adeptus Mechanicus tried for years unsuccessfully. Thus it's both a Death World and Penal World, as well as an Adeptus Mechanicus Harvest World where the Biologis Branch sees the unique compounds as extremely valuable. If riots occur inside one of the filtered domes, then the Techpriests simply turn off the filtration. Home to the beautiful, yet deadly, Phyrr Cat.
• Jemadal, The planet that the Raptors crashed their entire chapter into after getting blown of course in a warp storm. A planet covered in dense slimy jungles with carnivorous plants, swarms of voracious predators, and enough humidity in the atmosphere to suffocate a Space Marine. Since the entire Chapter was stranded there for 7 years, they got pretty good at jungle warfare and basically turned into Space Marine Catachan.