I'm only learning the modding business, but I think I made a little discovery. I'm putting it up here for those who may not know about this. I was trying to get a Summon Creature effect for an invisible creature, and it kept appearing in clouds of smoke. It had no avatar, but the smoke was there. Apparently it's an integral part of the Summon effect. Well, I looked at Animate Dead, because it uses a different visual, and I saw that instead of a direct Summon Creature effect it calls on Use EFF File instead.
The EFF was a Summon Creature again, but it had more fields, and in Resource 2 it had SPRAISE. This it the climbing-from-the-grave effect in the list of Effects here: http://gibberlings3....pcodes/bgee.htm (the version for BGEE). Apparently if you put one of those effects in that field, it overrides the default smoke animation. Well, I didn't want a skeleton either, especially since these set effects come bundled with sounds, a sinister one in this case. So in that list I looked for an animation that would do nothing, and I found a couple. By typing in SHSKULL I was able to get the creature appear truly invisibly.