"What is in there is more dangerous than warpstone - what ignorant scholars once named wyrdstone to deceive themselves and cloak their own fears. This is poison and pestilence cast into stone by the foulest of magics. To touch this is to touch death."
- —Jeremias Scrivner of the Grey Order[1d]
The Wormstone, as it was called by the Skaven, was a large chunk of warpstone that had been altered by the arcane alchemical abilities of Clan Pestilens to produce rampant disease, even among other ratmen. It was the colour of swamp slime, crossed in veins of black, and emitted a sickly yellow glow instead of the signature green associated with most warpstone.[1a]
The bulk of the artefact was pure warpstone, but it also was comprised of other unknown substances. It emitted an evil aura that repulsed even the Skaven. This was because the Wormstone had powerful disease-causing abilities as whoever touched its surface would find themselves infected by glowing maggots, likely the namesake of the stone.[1a]
These "worms" were actually the warpstone residue that was found within all living things of the Known World ever since the Great Catastrophe had first unleashed the energies of Chaos across the globe. While under normal circumstances the concentration of warpstone in any object or living thing is too diluted to cause harm, the Wormstone acts as a lodestone, twisting the residue into deadly tendrils of Chaos corruption and mutation. As the bodies of Skaven are even more saturated with warpstone residue than those of surface worlders, the Wormstone proved especially deadly to them.[1d]
An ancient artefact, the Wormstone stood about as high as a Skavenslave. Unearthed from the mass-grave of Clan Mawrl in Under-Altdorf many years after that clan's destruction ("a thousand breedings" according to the Skaven's imprecise understanding of time) by Grey Seer Skabritt, there began a fierce battle between the Skaven clans to control it when its existence became widely known.[1a]
According to Nurglitch VII, the leader of Clan Pestilens, his clan had originally crafted the stone by altering normal warpstone through the use of Pestilens alchemy. Later, Grey Seer Thratquee revealed that the artefact was given to Clan Mawrl by Pestilens as a gift for that clan's loyalty to the Pestilent Brotherhood during the Second Skaven Civil War.[1b]
However, it was only death that Pestilens had given their ally and the Wormstone's infectious power killed every member of the clan, from the most powerful Skaven Warlord to the lowliest whelp. Once the infection became widespread in Clan Mawrl, the other clans of Under-Altdorf conspired against the infected clan to save themselves and buried every member of Clan Mawrl alive alongside the deadly Wormstone in order to stop it from destroying the rest of the Skaven population of Under-Altdorf.[1b]
The Wormstone proved powerful enough to tip the balance of power sharply in the direction of whichever clan made use of it. According to Thratquee, it proved to be potent enough to cow the Council of Thirteen themselves. [1c]