In Greek mythology, ambrosia was considered the food or drink of the Olympian gods, and it was thought to bring long life and immortality to anyone who consumed it. It was often linked to nectar, the other element that the gods consumed; usually, it was thought that ambrosia was the food and nectar was the drink of the gods. In general, ambrosia was only consumed by deities; when Heracles achieved immortality, Athena offered him ambrosia; while when Tantalus tried to steal some to give to other mortals, he was punished for committing hubris. Whoever consumed ambrosia no longer had blood in their veins, but another substance called ichor.
One of the myths about Achilles’ immortality has it that his mother Thetis anointed him with ambrosia when he was born and then passed him through flames, so that the mortal elements of his body would be consumed. However, Peleus, his father, found out and stopped her; this caused Thetis’ rage and left without managing to immortalise Achilles’ heel. As a result, that was the only vulnerable spot in his body.