Soul Slay is an important Combat spell, one of the more-important ones to look out for. It's a Level 5 Thaumaturgy that requires 3 and (unless you're under-leveled) consumes no Gems on use. It's effectively a more-impactful Mind Burn, with permanent consequences.

Like with Mind Burn, the target suffers nothing if their Magic Resistance wins out against their foe's Magic Penetration in a DRN roll. It also doesn't work on the Mindless. On the other hand, if it works, the target is immediately killed. This is no ordinary kill, for it splits the target's mind from their body down to a spiritual level; the target stays dead, even if they had extra lives, and even if they possess (up-to-that-point) working Immortality.

Oh, Pretenders can still be recalled. Apparently faith can fasten minds back to their bodies. On the other hand, they will lose a point of Dominion or a Magic level as normal, whereas this wouldn't happen if they came back through their immortality. There is a soul slayed Affliction, but that's just a marker for peeking at the Immortal normies in the Hall of Fame, to remind the audience that they will stay dead. Immortality starts functioning again after the soul-slaying is overcome.

Oh, Unique ritual summons can be re-summoned as well! It's no big deal. I don't know who they have that reattaches their minds to their bodies for free, but they do a decent job.

Unfortunately no; some items, two Doom Horrors, and at least one Pretender form can soul-slay in melee. On the bright side, Soul Slay is the easiest and most-prevalent method of spooking Immortal foes. Make of this what you will.