What exactly does the "Spider Attack" ability do?

It's under the Undead > Units section. As far as I can tell it just gave me an odd looking "missile shooting back at caster" thing.

It does that, makes the projectile return to the caster.

Hmm... Is there any way to utilize it in a special way, though?

Boomerangs!

EXCELLENT. So we can use it for all sorts of eye candy shenanigans without any repercussions right?

When I played with it, the attacks' missiles travel at a specific speed, regardless of the unit's missile speed setting. Changing it in the spider attack ability did nothing, although missile art worked (so you can have a different model for the outgoing and returning missiles). Also, the spidery art when the missile clings to the target is specified in a Buff/Effect, even though the ability itself does not refer to it.

You might want to test what happens when combined with orbs and/or buff placers.

There is a minor issue, IIRc, and it is that it does not display a ribbon (that effect trail models like the ziggurat missile has) if the model has it.

its damn hardcoded and does not work with orbs.

Perhaps with multishot or bounce?

If it doesn't work with orbs then it won't work with those either.