Impossible Inferno

Six damage at instant speed is a lot of pointed removal, enough to kill essentially any creature a fair deck fields, but five mana is a steep price for a burn spell of this size. The delirium rider recovers that overpay: assemble four card types in your graveyard and the kill spell refunds you a card, exiling one from your deck and handing you a window to play it that runs through your next turn. That structure asks a specific question of the deckbuilder, namely whether you can reliably reach four graveyard card types (a land, a creature, an instant, and a sorcery clears the bar; artifacts and enchantments only widen the margin) by the point you want to be spending five mana on removal. With the condition off, you have a slow, expensive one-for-one. With it on, the exile-and-play clause turns a reactive spell into tempo-positive card advantage without ever shifting off instant speed, so it fits the same open windows any combat trick or end-step answer would. The design is less about the six damage, which is generous but incidental, than about rewarding a graveyard already stocked to do other things, letting a self-mill or spell-heavy shell fold premium removal into an engine it was running anyway.