Brainstorming ways to get a Save or Die within Mathilde's paradigm:
- Baleful Ethereality: Remember the grass we killed when we were learning Shroud of Invisibility? And how the answer to "can we choose to fall through the ground when we're using Substance of Shadow" was "yes, but then you're stuck until you hit the mantle and you die"? Do that to the bad guys. Even if it turns out that making them fall into the mantle is the "boosted" version, the non-boosted "take some enemies out of play for a while and they die if they mess up while they're ethereal" should be pretty good.
- Vanish Into Mist: Things tend to be hard to find when there's a ton of fog or mist out. Sometimes they don't come back. Do that to people. No dragging them screaming into hell, no glowing purple doom ball. Anything in the fog is just... gone... when the fog burns off, having joined it in going to wherever it is that fog goes when it clears.
- Mistify: Just vaporize someone. Simple as that.
Edit: Those are all pretty similar, suggesting they could be interpretations or conceptual foundations for a single spell. Concretely... Pall of Darkness to make Substance of Shadow work, then hit everyone with Drop to make them fall through the ground or dispel the spell really messily so they don't come back from Substance of Shadow in one piece?