A gaming buddy of mine had something of a thought on the Soul Bind spell yesterday. He brought to my attention that while the spell has a listed value needed (a black sapphire worth 1,000 gp per HD of the target), it doesn't really specify that the sapphire in question needs to be "empty".
His angle to this spell is that instead of only being able to hold one soul, it can hold several, but the gp value determines the strongest soul it can hold. As an example of this, he showed me the book Start of Darkness, from the Order of the Stick series. On pg 106, the main villain, a lich, used Soul Bind to trap the soul of his recently-killed enemy, an epic-level wizard, inside a black sapphire. But the thing is, he had already trapped that wizard girlfriend, a high-level druid, inside the sapphire beforehand.
So that's two souls in one gem. Now, either that sapphire was big enough to hold all the HD of their combined souls (a bit of a stretch, but it was bigger than his hand, so, maybe), or it was big enough to hold the epic wizard, and the druid was just a bonus.
Now, either the author (Rich Burlew, great writer) was doing it by taking some liberties with the source material for the sake of plot, or he's found a genuine loophole in the spell.
I've read quite a lot of RPG books based in the DnD system, and there has *always* been spells, creatures, magic items and even materials that can trap a soul upon death, but very rarely (at least in my experience) has there been any such means that could hold multiple souls at once.
Now, there's nothing in the spell description that denies this possibility, but I'm a little unsure about how to handle it.
On the one hand, it gives the players an advantage in that the don't need to do as much hunting for resources if they, for whatever reason, want to use the spell multiple times. But then, NPC's gets the same advantage, and seriously, how easy is it to find a sapphire big enough to handle the beasties on the higher end of the CR-range?
On the other hand, thematically I don't think it'd be that much of an issue. It'd basically be like the caster putting all their eggs in one basket, which has its own shares of dangers. And there are plenty of examples in modern entertainment of methods of holding multiple souls (I'm mainly looking at the Darksiders series here).
Imagine for example a necromancer that had used the above method, and had a very big sapphire holding several souls of his enemies. If that gem was to be smashed, or for that matter crumbled to dust because he simply miscalculated how many HD a new target had, all the souls inside would be freed at once. And if I was feeling like some poetic justice, I'd have all the now-freed souls manifests as ghosts and promptly proceed to kick the necromancer's ass.
Any input on this matter would be greatly appreciated.
As far as I read the rules, binding two (or more) souls in one gem seems to work fine:
Focus (F): A focus component is a prop of some sort. Unlike a material component, a focus is not consumed when the spell is cast and can be reused. As with material components, the cost for a focus is negligible unless a price is given. Assume that focus components of negligible cost are in your spell component pouch.
Flavorwise, I agree that it makes more sense to use different gems for each soul, but the eggs-in-one-basket analogy is also good; one wouldn't want to risk releasing all of one's souls by one single gem being destroyed.
Although this does make soul-hunting easier :)