The Forbidden Fruit, often envisioned as an apple, was a divine fruit that became a tool of evil when the Serpent successfully tempted Eve into eating it, which was a grave sin as the Forbidden Fruit, true to its name, was the sole fruit in all of Eden that God forbade humanity from eating.
Eve in turn tempted Adam into trying the Forbidden Fruit and at once the two of them became aware of their nakedness and tried to hide themselves out of shame, when God arrived to visit them they became afraid and tried to hide.
However, God found them and thus, this began the event known as the Fall of Man. It is said the Tree of Knowledge of Good and Evil that bore the Forbidden Fruit still exists in the Garden of Eden, but God tasked a powerful angel known as Uriel to guard the gates of Eden so that no mortal may enter it, as was part of his punishment for Adam and Eve's transgression.
The Forbidden Fruit is since then become a task to the corrupt for them to acquire and harness, as they believe that by eating the fruit, it will not only give them everlasting life, but divine power as well.