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 Imperial citizens 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. Note that the planet still has to technically be colonizable to be a Death World; a gas giant may have 4,000 km/h winds and rain glass, but you can't colonize a gas giant. In short, Death Worlds are 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. Canon floats two reasons. One: some of these worlds were colonized many ages ago and only became death worlds later on, such as Krieg (which was nuked long after it had been settled) or Chemos (before it was properly Exterminatused). Evacuating a planet is nigh-on impossible so the Imperium just tells the people to deal with the new problems. Two - liberated from Frank Herbert's Dune: if you can survive what a Death World throws at you, then you're badass enough to become an Aspirant into the Sardukar Space Marines. The leftovers? They're a veteran, self-replenishing recruitment pool for some of the most badass Regiments of the Imperial Guard.

Or maybe the world is too powerful for the Imp**BLAM**

Notable Death Worlds

• Baal (and its moons), homeworld of the Blood Angels.
• Barbarus, homeworld of the Death Guard (subjected to Exterminatus sometime between the conclusion of the Battle of Beta Garmon and the start of the Siege of Terra by the Dark Angels who outright Alderaan-ed it.)
• 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. Daemons themselves are literally terrified of this itchy hellscape. And with good reason.
• Fenris, homeworld of the Space Wolves, both before and after Magnus came along and made a big mess when he magicked all over the place.
• Medusa, homeworld of the Iron Hands. Or, even worse, there exists the worrying possibility of Medusa actually being a fucking dormant Tomb World. Poor Iron Hands just can't catch a break.
• 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.
• Phaedra, a major part of Peter Fehervari's sub-setting, The Dark Coil. It's full of predators, bugs, flora, and fauna that will kill or infect you in the most painful way possible. If you don't get taken in by its unusually subtle Chaos corruption, that is. It's not as outrageously lethal as the other planets on this list, but the sheer ease with which humans and Tau alike painfully die or go insane on the planet more than qualifies it here. It's even noted that it's "almost" classified as a death world, but this classification ignores the various reality warping horrors the world subjects it's visitors to.
• Phyrr of the Calixis Sector. Yeah the air and water is clean, and the whole place looks normal, but the spores and animals are lethal there as they have some bizarre gene-toxin that kills mature humans in an instant. The Adeptus Mechanicus have tried for years unsuccessfully to find an antidote. 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.
• Settler's Bane, an aptly named planet where the flora and fauna is so lethal that the Feral Orks infesting the place are regarded as a "minor nuisance" by the human inhabitants of the planet. Imperial Guard regiments raised from the populace are known for being pretty eager to get into close combat, and often wear Ork hides over their uniforms.
• Luther Macintyre IX, a desert planet known for sandstorms capable of flaying a human's skin from their bones. The homeworld of the subterranean and rather nasty Ambull, which has since been exported to plague dozens of other Imperial worlds. It is also known for having Mica-Dragons, the teeth of which were used in the creation of Angron's chainaxes Gorechild and Gorefather. As if all of this didn't make it a horrible enough place, it might also be a Necron Tomb World. How can a tomb world be even worse you ask? Well, as one of the most recent Dark Imperium novels puts it, "Chaos is and has been the single greatest threat to the existence of Mankind since its inception... But in all that time, the Necrons had remained dormant. Had."
• Phyrr: a planet which has an ecology that for some reason is extremely toxic to humans. A little Phyrr pollen on the wind can turn a man into a pile of slime and bones.