Where there has been a concentration of undead for several years, a strange, inky black substance can sometimes be found, pooled in hollows in the ground. If you picked it up, you would find that it pours and moves like a dark, viscous liquid - but it doesn't feel wet. At first it feels cold, then your hands become numb, then strength begins to ebb from your limbs. The solitary adventurer may be overcome at this point, but if you have friends (or at least companions) with you, a healing potion or power will restore you to health.
The unscrupulous soon discovered that this substance, when suitably processed, makes an effective and deadly poison. They called it "raw doom" and set up Workshops of Doom, sometimes even Factories of Doom, to refine and mould it into a more convenient form. A drop or two in a cup of wine induces death while the drinker is sleeping that night. A weapon coated with it strikes through armour and is only stopped by certain white fabrics which have been blessed by sacred rites.
The gathering, processing, selling, buying and owning of both raw and processed doom is illegal in all human, elvish, dwarven and goblin dominions. Nonetheless, there are some desperate souls who traffic in it. They call themselves Doom Merchants, or sometimes Merchants of Doom. The Doom Merchants venture deep into necropoleis, cemeteries and ghost towns, wherever they can find undead, and gather large quantities of raw doom. This they carry in large black bags - Bags of Doom. Their customary costume includes thick white gloves and masks for protection, though the black and white vests they also wear are thought to be ornamental.
By all accounts they make substantial sums of money plying their trade, but slowly lose all colour in their skin and hair, then within a few years become weaker and weaker until they collapse. Rumour also says that after death, they themselves turn into undead, with a shadowed, striped, spotted form that breathes out thick clouds of doom.