Death in the World can come in any number of ways. If someone dies and nobody burns the body the soul remains trapped for weeks, months, or even years, unable to fly free and search for a way to the gods of the afterlife. It remains prey to ghost-catchers, night-stalkers, and demonologists, who respectively enslave souls, eat them, or use them to pay demons for their services. There are enough ways for magicians to communicate with the dead that the exact nature of ghosts is well-established.
While preferable to have the whole body, it is impossible to burn a body properly without at least the heart and the head at the absolute minimum. It is also recommended to have a necromancer or theurgist on hand to help guide the soul.
A standard part of execution for criminals is to remove their heads and post them, thus denying them the possibility of a proper pyre.
It should be noted that necromancers can speak to some of the dead, but not others, and the reasons for the differences are unclear.
There is no known way to return an individual who is dead back to life.