Potion of Youth

The Potion of youth is a Noble Phantasm that returns the user to their youth that is stored in the Gate of Babylon. It is kept in a small, translucent bottle with a lustrous, beautifully-ornamented design made of an unknown material resembling china or crystal. It is the herb of immortality that Gilgamesh had sought after the death of Enkidu. Seeking immortality to avoid his own inevitable death, he sought out the sage, in Kigal, but learned that the sage's immortality was obtained from becoming a god. Rejecting it because it would remove his desires as a human, the sage instead told him of the root of a herb that grew in the deep of Kigal that was the true secret behind immortality without seeking the mercy of the gods.

Gilgamesh eventually reached an epiphany that no longer required him to be immortal, and he believed consuming the herb would cause him to become a plant like Utnapishtim. As it was still a rare treasure that he had planned to obtain even before his desperate quest, he took it only to keep it as an exotic wonder to decorate his vault. It was later stolen by the snake, but it was the "restoration of youth" rather than immortality, granting snakes the property of shedding their skin. He later returned to the deep and recollected it even after giving up immortality simply to complete his collection.

Drinking it returns him to the form of a child, drastically altering both his appearance and personality. It is used in cases where he would need it to tolerate and blend into the society around him, such as spending ten years in the modern world after the Fourth Holy Grail War. He can also use it in a case where he views something as a childish game like the False Holy Grail War, claiming that he would spend his time in leisure until an enemy worthy of his full strength appears. It allows for him to keep all of his memories, but he is disconnected from his child-self due to the vast differences between their personalities. Though his child-self knows of his later actions, he cannot comprehend his adult self's actions, and his adult self has little knowledge of his past. He has the ability to forcibly reverse the effects when facing an enemy he feels is worth fighting.