The precise nature of mouldstone is not understood, but it appears to be very alien and magically unstable, and unshielded mouldstone is alleged to mutate living things. The Apep apparently made use of this effect to create abominations as living war machines, and eventually weaponised the mouldstone to build null bombs. Shortly after the invention of the null bomb, Sil’ess civilisation ended.
A shielded casket of mouldstone was retrieved from a site in Clark Hills by Wayne Epping, leading to pursuit by Souther mercs employed by the Scales of Fate. The party sold the casket to Riffkin, earning the ire of Acher. Steig’s advice for dealing with the mouldstone was to either punt it into a rift or destroy it with a sufficiently powerful attack, which may have devastating results.