Classifying undead monsters is a continually evolving task. Necromancers are constantly experimenting, curses upgrade powers and degrade physical forms, and environmental factors regularly tweak the magical factors that build, animate, and repair these creatures.
A Shade of Interaction
Different schools of thought generally resolve this tricky issue with two sliding scales: corporeality and interaction.
Many “ghosts” are recordings of a moment in time. They lack memory and the ability to interact with the living world — they are ethereal projections, typically bound to a single physical object. Their haunt cycle repeats endlessly, forever unchanging, even in the face of the destruction of their anchor. These shades are low-powered, relatively harmless, and completely mindless. While many heroes seek to “free” them of their eternal prisons, an enjoyable strategy is to seek out their anchor and bring it into a haunted place of power. Over time, enough dark energy feeds into the anchor and the shade will become a full fledged ghost, able to travel and wreak terror across the land.
It should be noted that not all ghosts or spirits started off existence as a shade. Many people and creatures pass from life fully into undeath, able to remember and communicate immediately. This does not, however, promise the ability to move objects or be touched by them.
A Touch in the Dark
Beyond sentience, the other major pillar of undead measurement comes from “corporeality” — the measure of physical form and, in this instance, the ability to interact with physical and magical matter. We have reached the point where Undeath gets very tricky.
Fully Corporeal: At the core, corporeal undead can touch, lift, push, interact — relative to their physical strength — with objects. For instance, “the skeleton picked up the bottle of poison.” In this matter, they can also be interacted upon, “the skeleton was hit by the water balloon. It is wet.”
Semi-Corporeal: Some undead can only be physically touched by,
or physically touch, certain items. The most common limitation is silver or magical objects. Junior necromancers are often employed to lightly enchant household items for undead moving into a new home. The spell is relatively simple and inexpensive, so some adventuring undead learn it themselves and use it during their travels.
Situationally Corporeal: A blend of the above, the monster’s physical form is changed by external factors, such as moonlight. This is very unusual and requires a significant amount of magic.
Non-corporeal: The monster cannot touch or be touched by any object or force. Long term, this is the rarest state of undeath, but not due to any natural factor. The spell to introduce corporeality, a more comfortable form of existence, is not very complex. Non-corporeal undead often seek out necromancers or their schools to help, in industry terms, “pull them together.” Fees typically range from a few pieces of gold or haunting a particular location for a year or two.