Grave Magic

There are some spells which have been developed by the spirits of mages who refused to pass on into the afterlife. Instead, these ghostly mages continue their lives in the ethereal plane much as they had lived in their everyday life-perfecting rituals, tweaking the right combinations of spell components, and designing invocations for maximum effect.
Of course, many of these spells are totally awesome, but there are no mortal equivalents for the components, words, environmental conditions of the ethereal plane. To gain access to the spells, certain folk will unearth the remains of one of these ghostly mages, which act like tethers for their spirits. They build elaborate cages and cases for the corpse, binding it more closely to the body so it can be dragged or pushed or carried around. These always include a set of special goggles that look like glasses with one regular and one arcane lens each, allowing the dead and the living to see through one another’s eyes, coordinating the casting of the grave magic. The living wizard can then request, compel, or trick the ghostly mage to cast one of these spells. However, to transmit between the ethereal and material plane, the spells need the portal of a soul which has got one foot on either side of the fence, so to speak. The terribly wounded, recently dead, mummies and zombies, and beings like other restless spirits will do, as will certain magic items created with necromantic magic or the semi-preserved remnants of magical animals. Thus, grave magic is often most effective in the midst of an active battlefield, in a haunted graveyard, or at other sites where the veil between the ethereal and material plane is worn thin. 

Spells of grave magic might include:

Visions of the Grave – the target is cursed with eyes that only see the ethereal plane, though their body is locked in the material plane. Coordinating actions between one plane and the other is dangerous, since ethereal structures are often more complete—broken bridges, stairways into nothing, etc—than their material equivalents, and often material structures have no spiritual double. Not to mention this is effectively blinding in a battle, where you can see your new enemy but can rarely do anything to effect them.

Grave Touch – the victim is marked in the ethereal plane, and subject to a one-way mirror effect of damage and sight: ghosts can see and hit them, but not vice versa.

Grave Fireball – a ball of seething necromantic energy, shaped like a fireball but made of shadow and propelled by the cold wind of the grave, explodes forth into the material plane. In addition to a fireball’s worth of necrotic damage, its’ mere presence is enough to shake the will of the living, as visions of the vengeful dead fill their thoughts in the wake of the explosion. Fireball with will save against fear, with the possibility of lingering madness.

All of this stuff gets more fun with a character who can see into the ethereal plane and help his friends dodge, duck, dip, dive, and dodge out of the way of the grave magician. Breaking the link between the living wizard and the spiritual one becomes a focus of a fight. Or perhaps convincing the dead caster to fight for the PCs instead! Or, using your own speak with the dead spell to fight grave magic with grave magic and bog the dead wizard down with junk on his side of the veil. My examples are all vaguely evil and battle-related because I am looking at heavy metal covers all the time, but I bet there are tons of cool effects that might make more sense in a necropolis or for clerical versions of this where the caster might use this new magic to protect spiritual sites and allow tortured souls to pass peacefully on.

Inspired by:
Skeletonwitch Album Covers/just the name skeletonwitch
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A story I heard on NPR about musicians who play songs based on real-time improvised choices of musical phrases chosen by ex-musicians who have become paralyzed but can use specialized brain-scanning computers to relay their choices to the non-paralyzed musicians.