Magic that involves the dead is called necromancy, which can be its own type of magic in our world. Communication with the dead means understanding how the afterlife works. After all, if spirits are simply obliterated, bringing one back or interacting with it is impossible. The nature of an afterlife might also aid or inhibit the recruitment with or contacting of the dead. Would a Satan-like figure make that contact easier so that more hell is loosed upon the Earth? Would a God-like figure inhibit it? Or would God allow interaction because it might be benevolent? Having a justification and multiple barriers can make scenarios more plausible.
Communication can mean the equivalent of a long-distance phone call, meaning the spirit never leaves where it resides. This seems easier than summoning the spirit to cross boundaries; this is where inventing afterlife barriers figures more heavily. A third option is raising a body from the dead and restoring the soul into it, a more intense experience for both necromancer and victim. This is especially true if the target does not become undead but a living person once again. By dividing these concerns this way, we can begin creating a system of magic for necromancy.
We should also understand what death has been like, as this impacts the mind or emotions of anyone thus contacted. That, in turn, will affect behavior, including how cooperative they are, though perhaps our necromancer can force obedience – at least until he’s incapacitated and the undead either get free and go their own way, return to the land of the undead, or take revenge upon the necromancer who awoke them into servitude.
We can decide that the undead have acquired unlimited knowledge, increasing their value, or only have firsthand knowledge just as in life. The Roman poet Ovid speculated that an underworld marketplace exists where undead exchange news and gossip, which not only expands what undead know, but creates a place where the living can travel. Perhaps knowledge isn’t the only item traded here.
Necromancer rituals can last for hours, days, or even weeks, and sometimes use moon phases or the placement/configuration of other heavenly bodies. These often take place in or near graveyards, tombs, and mausoleums, though the churches of evil deities are an option. While words, gestures, or materials might be involved, these rituals often involve a circle drawn in blood, holy water or an equivalent, salt, or writing. These serve to protect the practitioner from possession, assault, death, kidnapping, and more, from the very forces they seek to control a threat to their lives and soul. These circles can also concentrate magic power where the target dead will be imprisoned after summoning; bringing a potentially malignant being to this realm is inherently dangerous and unpredictable, making this precaution wise.
Sacred talismans, relics of the dead, and body parts such as bones may be involved, whether worn, laid on circles or altars, or used during gestures. They could be mutilated, incinerated, or otherwise destroyed, which naturally means they can’t be reused, which places emphasis on success the first time. Placing a limit adds urgency that benefits stories. The practitioner may be wearing the deceased’s clothes or an item that held importance to them.
Foods that symbolize death, and fluids like blood, are often present. These are consumed by either the necromancer or enthralled spirits. We can invent a reason for this, such as giving a spirit more substance and therefore power over a world of which they are no longer a part. Perhaps consuming it binds them to the world, to the necromancer, or to do the deed for which they’re now recalled. Can the necromancer force them to consume it?
A more extreme practice involves sacrifice, which could be an item, animal or human (or an invented species). This might mean a life or the soul, too. The victim could be the infamous virgin, but an excuse for this might help alleviate this cliché to something more palatable. If we state that innocence, more than virginity, can counter the evil forces involved in necromancy, or fool a guardian of the underworld into letting our necromancer access a spirit by suggesting we have pure intentions, this concept makes more sense. Youth also seems plausible for younger people’s strength, but a baby has little, so a teenager might be ideal.