Antimortallium is a metal bar (of which only one instance exists in Nehellium) that was originally discovered by the Nyphilians, who claimed that it was capable of permanently destroying Immortal cores. Their empire was eradicated by the Immortals.
The Trattorians happened upon it later on and discovered its much more useful property of generating neutrinas when placed in a particle accelerator, providing fuel for intergalactic voyages and galaxy-destroying bombs. Upon hearing of the old rumor, the Trattorian Black Box Bureau conducted a test to determine if antimortallium could actually kill an Immortal permanently.
The results were, at best, inconclusive.