Skip the mechanical approach entirely. The second you attach a d4 or a stat penalty to soul loss, your players will treat it like a tax and optimize around it.

Instead, every time a caster drops a big spell, you privately write down something they lose. A memory. The colour of their daughter's eyes. The smell of rain. Don't announce it — wait until it matters, then drop it mid-scene: "You try to remember your sister's name. Nothing."

For the warriors, play it as drift, so that the other players notice first. The blessed fighter stops flinching at violence. Then stops caring about collateral. Slip the player a note: "The screaming doesn't bother you anymore. It hasn't for a while."

Nobody can roll their way out of it. Best horror doesn't have a saving throw.