Death worms have the feel of meta-GMing about them, an everything-but-the-kitchen-sink suite of abilities (Acid breath! Electrical jolt! Poison bite! Venomous skin! Corrosive blood!) and related defenses that seem too blatantly the result of design rather than biology. Still, they have a name liable to strike fear into PCs’ hearts (even if the necromancers grumble about anything labeled “death” that’s not undead) and they’re more manageable throw at a party than the Gargantuan 200 hp purple worm.
A prophecy says a future leader of the oppressed Urgs will be a golden-haired boy who will first die three deaths: by tooth, by lightning, and by poison. When such a youth is found, a party of Urgs kidnaps the 14-year-old. Their aim is to stake him in the desert to let the death worms hurry the prophecy along.
In the night, a gnoll tribe surrounds a party of adventurers. At dawn, they offer the party a choice: defeat the mysterious force that has been harassing their hunting parties…or die. They have not yet ascertained the cause of their pack mates’ disappearances, but they superstitiously assume (wrongly) that a pack of metal-armored humans will have better luck.
Death worms are solitary creatures—in the deserts and badlands. But on the steppes, where prey is more plentiful, they sometimes gather in larger numbers. The khan’s horse lords (Advanced Player’s Guide 147) must learn special tricks to fight these magical beasts. And sorcerers of the fire bloodline, even while shunned by their tribes (the grasslands being so prone to wildfires), are nonetheless housed apart and tended by the khanate in the expectation that they will bring their flames to bear if death worms are sighted.
—Pathfinder Bestiary 2 76
Edit: Reader Landon writes:
The Death Worm is a cryptid, a modern legend from Mongolia called Olgoi Khorkhoi. The reason it has all sorts of crazy abilities in Pathfinder is because they're all from different forms of the legend.