Achilles was the half god, half mortal who was the hero of the Trojan War. His mother was the Nereid Thetis, who is known for her many interventions in the affairs of both gods and mortals. His Father was Peleus, a mortal, but it took a while for his mother and father to get together. Nereid had many offers from the gods Zeus and Poseidon, who were brothers, but she refused both of them. Some say she refused Zeus because Hera had raised Thetis when she was a child, but this angered Zeus and to punish Thetis he commanded that she would not marry a god but a mortal. Thetis was not happy to wed a mere mortal and complained that it was an insult to her status. So when Peleus tried to claim his bride she refused him and changed her form, first into a bird, then a tree, and finally into a tigress, scaring Peleus letting her go. Proteus, a son of Poseidon, instructed Peleus to not let go of Thetis no matter what form she took. When Thetis turned into fire and water Peleus still held on, and, in admiration of his perseverance, she finally agreed to be his wife. Ironically, the wedding of Thetis and Peleus was the cause of the Trojan War, the war in which Achilles died in. Seeing how Achilles was half god and half mortal, his mother wanted him to me immortal. So she took him to the river Styx , which was a river found in the underworld and anyone who bathed in it became invulnerable. She dipped Achilles in the river, holding him by his ankle, which was eventually the cause of his death. Some other theories on how Achilles became immortal are that Thetis would hold him over the fire to get rid of the mortal elements inherited from Peleus, and that in the daytime she would anoint his body with Ambrosia. Peleus was said to have caught Thetis doing this to their infant child and when he did he got very angry with Thetis and she ran out on both of them. Peleus chanced upon his infant son in the fire one night and freaked out on Thetis. In disgust...
