Creating undead in D&D is evil, except where its not. Its descriptor judges if it is evil or not. Baelnorns (Either created through elven high magic or created via one of the elven pantheon) and Shadowdancer created Shadows (which are undead shades) are two great examples of none-evil undead creatures. Revenance (Hoarite Spell in 2e, a general Paladin/Cleric/Blackguard spell here) summons a vengeful undead shade and does not contain the evil descriptor (thusly, not evil by D&D's rules).Rhifox wrote: Hovering Skull has the Undead type. It is harmed by Positive energy and healed by Negative energy (which means it is powered by negative energy). I killed one of my skulls by using a Cure Moderate Wounds wand on it, while I can heal it with Negative Energy Ray. It is not just a levitating object where the object just happens to be a skull (such a spell would, frankly, be evocation, not necromancy), it is an undead and therefore it is an Evil spell. In DnD creating or summoning undead in any fashion is Evil.
As Whistler (and so many others, as this question is asked time and time again) pointed out:
Does it have the evil descriptor?
Yes=Evil Spell
No=Not Evil Spell
This is the general rule of thumb in D&D, and it appears to be the general rule of thumb for the DMs here (DMs, feel free to correct me if I am wrong on this).
This should really be stickied in Tips and Tricks. That whole "how to tell if a spell is evil" thing. If that is indeed the DM enforced method.
As for the original question:
I haven't seen the spell IG or not, but I'm pretty sure it doesn't have an evil descriptor (else, someone would have likely mentioned that). It doesn't on the BG wiki, at least.
http://bgtscc.wikia.com/wiki/Hovering_Skull
Also, its worth noting that our characters can react as if the spell is evil. PCs/NPCs (except maybe gods?) shouldn't know what alignments are, and shouldn't know what alignment spells are. One man might see Baelnorns as horrid evil things! One man might have grown up around Baelnorns and see such undead creatures as "kinda-ok". Who knows!