…Ick. And I wrote it.
Wirestrictors
Generally speaking, necromancers prefer to bind dead human souls into items, instead of animals; mostly because it’s easier to suppress a human soul’s innate intelligence and ability to understand language than it is to boost an animal’s. In fact, it’s largely impossible to do that with animal souls at all. What you rip out of the body is what you get, and if what you got was something not capable of doing anything elaborate, you had better stick with simple.
Wirestrictors are pretty simple. Take one constrictor snake, rip out the soul, then shove it into a long, strong metal wire (showing no care in either the ripping or shoving: you want the snake soul upset). Then set a secondary spell that will render the snake soul ‘unconscious’ until a pre-chosen triggering mechanism is activated. Coat the wire in something that will hide the discoloration and reek of wrongness. Attach the ends to a brooch or gem or whatnot (not too strongly).
Congratulations, you now have a cursed necklace: when the ‘trigger’ is activated, the extremely agitated and unhappy wirestrictor will immediately start thrashing until one end or another breaks free. Once that happens, the wirestrictor will invariably tightly wrap itself around the victim’s neck, usually fatally. Wirestrictors are more or less built to self-destruct: the wire trapping the soul cannot sustain for long the internal damage caused by the trigger’s activation.
Keep in mind: this isn’t a nice use of necromancy. In fact, getting killed by a wirestrictor is a very messy way to go, and watching one work will probably panic about half to two-thirds of the people (depending on the campaign) witnessing it. The remainder will instead want to hunt down and kill whoever made the wirestrictor in the first place. Possession of one of these items is a capital offense virtually everywhere – including formally Evil-aligned regimes: this is an assassination tool directed against the rich and powerful. Any god or religion that likes snakes even a little is going to want to do horrible things to a necromancer who uses this spell. Shoot, there are necromancers who would kill other necromancers who they catch using this spell, out of sheer enlightened self-interest…