By positioning a creature against a mirror, and marking their forehead with their own blood, the necromancer entombs the creature inside a mirror. The mirror must be of a size to hold the creature. Spells that change the size of the creature prevent the creature from being entombed until the spell is worn off. Similarly, mirrors that are magically changed in size are unsuitable for this task.
Once a creature is held in a mirror, it is in a state of two dimensional semi-stasis. The creature does not heal, does not age, has no need of food, drink, or bodily functions, yet they may still move and speak (if they were able to outside of the mirror). Furthermore, those trapped in the mirror find that their three dimensional belongings no longer function in two dimensional space: books cannot be read, potions cannot be consumed, itches cannot be scratched, etc. Creatures with magical abilities (spells, gazes, etc.) will find that their spells are restricted to the mirror dimension in which they are placed. A basilisk would not petrify someone looking at it, nor could a mage within a mirror cast Magic Missile at someone on the other side or at the mirror itself.
Creatures inside the mirror are aware of what is happening on the outside of the mirror, being able to see, hear, and smell. Similarly, creatures on the outside of the mirror and see and hear those trapped within. Those inside the mirror may walk around within the confines of the mirror, and may come into the foreground (between the onlooker and their reflection) or stay in the background (behind the reflection of creatures).
If the mirror is broken or damaged (scratching the glass counts as damaged), the magic is broken, the mirror shatters, and the creature is freed, in the exact state it was originally imprisoned.
A shatter spell cast from within the mirror dimension will break the mirror. Spells that move between dimensions can be used as long as they do not require 3 dimensional space (A spell that opens a portal one walks through would not work, but a spell that shifts the caster from one dimension to another will). Teleportation has no effect.
A caster may not imprison themselves in a mirror.