Invulnerability
Associated with this is invulnerability, variously bestowed or acquired. In the tradition of Achilles, he was immersed in the river Styx by his mother Thetis, but the immersion did not extend to his heel, in which he received his mortal wound from the arrow of Paris.
Jason was rendered invulnerable in his battle with the giants that sprang from the sowing of the Cadmean teeth by being anointed by Medæa with the Promethean unguent.
Siegfried, the horny, made himself similarly proof from injury by bathing in the dragon’s blood, but one spot on his back, where a linden leaf had stuck, escaped. Through this only vulnerable spot he met his death, being killed by Hagan the Dane while drinking in a pool.
This probably is a poetic allusion to early employment of defensive armour, in which the back, as compared to the front, would be unprotected.