A cauldron is symbolic of the goddess Cerridwen, a goddess associated with rebirth, and have been useful to warring armies throughout the centuries. There is a spell involving ressureccting a corpse through a potion brewed within these magical items. Dead warriors could be placed within the brew and then be returned to life. The downside is that they lacked the power of speech, due to the lack of a soul.
This process has led to the creation of a perfect soldier. The resurrected corpse is completely obedient, willing to carry out his task without conscience or fear. These warriors can go back into battle until they were killed, in which case they can be resurrected again. Nations have used this magitech to bring back their fallen soldiers, reclaiming their bodies from battlefields and resurrecting them to send them back into battle again. This has led to variences on the procedure. Some countries have begun the practice of killing their soldiers in their prime years in order to resurrect them. Others have begun mixing and matching, cutting limbs and organs of the dead to mix within the brew, creating Frankenstein-like beings that have the best parts of each warrior.
The problem of this is seen when taken to the ultimate conclusion. Nations capable of this will no doubt seek to maximize this process and apply this in other ways, such as slave labor. It gets even worse when this tech trickles down into regular society.
I would like to keep the application this process only to warfare, specifically the resurrection of dead soldiers, and need to find a way to limit this magitechnology. How can I make this a reality?