I could see something like a shadow bolt working as a nice variant and I like your spectral skull wreathed in green flames as well, but the Dr. Frankenstein in me thinks that somehow electricity and the undead are not unrelated. Regardless, I would probably be inclined to accommodate any fireball-desiring player willing to put forth enough effort to imagine "spectral skull wreathed in green flames," though, especially because I love anything (within reason) that gets players creatively invested. 

The two replies following the OP nail my intent precisely: I was thinking Dr. Frankenstein all the way -- electricity being (at least in my imagination) an important component of several necromantic procedures. That being said, I do like the cut of your jib, Butcher (ghastly, spectral skull wreathed in green flames). ;)

It probably goes without saying the lightning is either sickly purple or ghastly green...

Handy Haversack wrote:

Blackadder23 wrote:

It probably goes without saying the lightning is either sickly purple or ghastly green...

According to Gary, mauve is also pretty evil.

I've always thought so.

Rastus_Burne wrote:

One thing I immediately thought of was Star Wars where the emperor makes use of lightning alongside the dark side ability to maintain longevity of life (and theoretically raise from 'death' should the user dabble enough in the arts). I know it's not swords & sorcery, but it's what sprang to mind 

This might be to your liking, then, Rastus_Burne:

Fingers of Leprosy (Evocation)
 
Level:  3
Range:  4” + 1” per level
Duration:  instantaneous
Area of Effect:  one individual
Components:  V, S, M
Casting Time:  3 segments
Saving Throw:  ½
 
The caster evokes this spell by cutting the undersides of his fingers at the completion of the casting, which mingles with a blast of purple netherforce drawn from the depths of Hades.  The energy cascades out of his fingers, and he plays it along the victim, causing 1-6 points of damage per level of the caster, as the energies rot, flay, and shred the skin from his bones (one-half damage if a saving throw versus spells is successful).  In addition, the victim must succeed in a second saving throw vs. poison or become infected with a random disease.