I love the title on this one. Though one has to wonder why a Necromancer would summon a Demon? Makes me think of undead infused with Demonic essences, powerful and unstable, hungering for the souls and flesh of the living. This a great oldie, thanks for dragging it out of its grave, Cheka.
I think this could even be used as the start point of a campaign, if the demon on remains long enough to scare the bejesus out of the PC's.
Other aftereffects of this occurrence could then make things move very quickly. Perhaps the PCs have seen far too much and there are those who would seek to silence them.