For Challenge # 105, as an answer to a broken deck including Knollspine Invocation.

I decided to assume this represented a deck with a critical mass of discard-and-reuse cards, so rather than delete the effect entirely, I deleted the recursion, in a way that doesn't shut off other graveyard effects.

Any suggestions for how this should be templated? I think it's important to hit both effects and costs in order to deal with eg. madness cards, but ideally it would look natural.

I don't think you need the second half. I can declare "I'm going to sacrifice a creature to pay for Fling", and then a replacement effect from Leyline of the Void can transform that sacrifice effect into sacrifice-but-exile.

So similarly, with just the first half of this card without the text-hacking, I can still activate Knollspine Invocation, it's just that the card I discarded will be exiled.

With knollspine invocation plus a flashback card, the first half is sufficient. But if they discard a madness card, can they use the madness replacement effect instead of the acolyte of the void replacement effect and get to cast it?

Even if so, I'm not sure if it's worth worrying about, but it seemed like a very likely way of abusing the invocation.