An older source, but The Broken Covenant of Calebais says this about ghosts:
Calebais is haunted by the ghosts of many of its former inhabitants, disembodied spirits still confined to earth because of their
actions in life, or because of a supernatural desire to complete
some task left unfinished. Many ghosts are dark, sinister souls,
living tormented existences beside the physical world without
being able to affect it in any real way. They are often confused,
sometimes violently insane, and may hate and fear the living or
take pleasure in making others share their suffering. Others may
be of good heart and seek only to put right what their passing
made wrong, or were made ghosts simply because they lived too
long in a magical aura. Ghosts are as individual as living beings,
and each has its own distinct personality.
The appearance of ghosts can also vary, from seeming completely substantial, so real that they even feel solid, to a luminous
haze in the air with only vague similarities to the human form.
All ghosts at Calebais can become invisible at will, hidden from
those who cannot perceive the spirit world, but only a few come
close to a state resembling solidity; most remain translucent and
rippled, like a reflection in deep water. As they spend their
Might Pool, they also lose their clarity, fading away completely
when exhausted.
Apart from coming to terms with their non-corporeal nature,
ghosts cannot learn or change; they are trapped with the mindset and skills they had when they died. They may be able to
teach others some of what they knew, but the distance of the
grave combined with the difficulty of maintaining their existence
means that they tend to forget what they are doing very easily.
Characters studying from a ghost must start over each day, reintroducing themselves every time. This will be frustrating for the
characters and for the ghost, and should be more trouble than it
is worth.
Many ghosts do not realize they are dead, and refuse to believe
those who say that they are. Only ghosts who accept their
nature and have become used to their incorporeal forms can
move through solid walls, ignore Fatigue penalties or wounds, or
in any way act as if they were not living, breathing humans.
Ghosts are solid to one another, of course, and interact in all
ways as if they were flesh and blood. For most ghosts, the physical world is as real as ever, except that people seem to play tricks
on them or do not always see them.
Ghostly magi can also cast spells, just as they did in life. The
ghosts of Calebais are described with some spells to represent
their specialties, to give you ideas of what they might do, but
they also have suggested Arts for spontaneous magic, and you
should give them other spells that you think that they would
have learned before death. These spells are not physical and cannot affect the physical world, but they are realistic illusions. A
Perdo Corpus spell might cause a flash of pain, while a Creo
Aquam spell might cause a powerful chill as ghostly waters rush
past. However, Imaginem, Mentem, and Vim magics still affect
the living normally, as can some Animal (animal minds) and
Ignem (light) effects.
Most ghosts also have some sort of magical power to affect the
physical world, even if it is only a mundane activity appropriate
to their duties and desires in life. Ghostly magi typically have
the power to make their spells “real,” affecting the living as if
their magic were cast in the flesh. These powers often cost them
some of their Magic Might to activate, but they gain this back
over time, and a day is typically sufficient to bring them back to
full strength. If they lose all their temporary Might points, or are
injured or killed by magic or other ghosts, they will return after
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a day or so, as whole as before, with no memory of what happened. Permanently destroying a ghost is extremely difficult.
However, every ghost has fetters that bind it to life. When
ghosts deal with the tasks they left unfinished, or address whatever circumstances prevent them from passing on, they fade
away forever. This should be a very rare occurrence, worthy of
more than a simple declaration; you should describe the process
in detail, portraying a vivid event. A ghost leaving the confines
of the mortal world is an awe-inspiring affair. Treat this scene
with the reverence it deserves, and make it memorable.
When they pass on, ghosts can leave raw vis behind. This vis is
often as ghostly as they were, tied to a spiritual memento of the
haunting, though there are magical ways to harvest this vis. A
Muto Mentem spell with a physical requisite can make the item
and the vis temporarily solid. Rego Mentem can transport ghostly objects, and Rego Vim can siphon the vis into some other
container. Finally, even if they cannot move it, magi can always
use ghostly vis simply by touching it.