The Hero is never entirely free from danger. When Achilles, Arthur or Superman is gifted with seeming invulnerability, there is always an Achilles' Heel, which both he and the reader know about; it is a mark of his valour – and of his fatal hubris – that sooner or later he exposes his frailty to the foe. The Dark Lord, on the other hand, may seem invulnerable until the last moment, and his defeat will come about through the valour of his foe, or through his failure to understand a Quibble, or through a discovery (as in the tale of Koshchei) of where that which makes him mortally vulnerable (in this case his Soul) has been secreted.
In Supernatural Fiction and Horror, the invasive element or being – the possessive Demon, the Revenant, the Vampire or some other form of undead creature – can normally be identified by a seeming invulnerability. Over and above the unholiness of this sensation, invulnerability almost invariably here – and often in fantasy – is a Plot Device. [JC]