He used to be called "the trickster" some 400 years ago, a nickname now forgotten but still relevant.
Musahhar has in his possession the dagger Ahya. Its blade, when sunken into a dead body, acts like a portal that lets an unholy spirit slip into the flesh and possess it for an unseen future. Striking a living creature with the dagger will exorcise the life essence and turn it into a restless, vengeful spirit; the body will start decaying as normal but cannot be used for possession (spirits call these bodies for "tainted vessels").
If a person asks him for help, but cannot afford any of his potions, Musahhar will gladly offer healing for free. Slowly whispering the word sudad over and over again into their ear, any life threatening injuries will heal up, all traces of poison will vanish, and all weariness will disappear.
As life returns to the poor person, all hope will quickly disappear as he or she realizes that the helpful necromancer just placed them under a spell. As Musahhar hands over the conceiled dagger (wrapped in leather), the cursed person immediately knows what has to be done: within a week, he or she must stab a corpse with the weapon, and bring a spirit into this world.
If the spell is not fulfilled, Musahhar will get up and start searching for Ahya; for him, the dagger shines like a bright beacon, and this helpful necromancer has all the time in the world to go searching for his blade.
(Original drawing found at Telecanter's. Poorly photomanipulated by yours truly.)