Ambrosia

Ambrosia (アムブロシアー) is a small magic fruit that appears in the world of Toaru Majutsu no Index.

Etymology
The name comes from the of Greek mythology, where it is often times referred to as either the food or drink of the Greek gods, that confers ageless immortality upon whoever consumes it.

Principles
In the Toaru Majutsu no Index, Ambrosia appears as a small cone-like fruit that, when consumed, gives whoever has eaten it ageless immortality, making them forever appear in whatever form they have upon consuming it. The effects of the ambrosia not only gives the one who has eaten it leave from of old age, but apparently any form of  as well, terrestrial or extra-terrestrial, as Ladylee Tangleroad personally experienced.

Moreover, anyone who has eaten the fruit can never use magic on their own, as it twists the cycle of their life force necessary for creating magic, as such one requires others to do magic for them.

It is unknown if the Imagine Breaker is able to negate the effects of ambrosia.

Background
During the, Ladylee Tangleroad came upon an injured crusader, whom she ended up saving. As thanks for saving him the man gave ambrosia to Ladylee, who tried to turn down his offer at first, but eventually yielded. She later ate the fruit, and from then became immortal.