It is coal like substance, useful for necromantic forging and rituals. It is a fuel that can be used by necromancers and amethyst wizards as fuel, though fire wizards might too.
The fire of Mortkhat cannot be extinguished by anything other than shyishian magic. Water and other means are not effective way of stopping Mortkhat.
Mortkhat is an alchemical substance created by hidden arcane means of true necromancy, magic that only Nagash and his inner circle know. Mortkhat was created by Mortarch of Sacrament the Arkhan the Black and by his Black disciples (current ossifectors of Null-myriad) and with the help of ossifectors from another legion like Xhandrosor. Mortkhat is a black substance that looks like coal but does not dirty your skin on contact. Touching it feels like touching ice that burns at the same time for the living, but for undead creatures or beings infused with shyishian magic it feels warm. When it comes in contact with pure amethyst magic or necromancy it will start to combust and burn everything in a few meters of the Mortkhat. This fire burns the souls of the living and transforms them into Nighthaunt, while the dead and their physical forms are healed and even spectres feel invigorated.
To create Mortkhat one must know the true secrets of necromancy that only Nagash, Arkhan, and a few ossifectors know. Though a lesser version of it could be created with insufficient knowledge... but at what cost?
