Ban vs. Red Demon

Ban vs. Unnamed Demon was a battle fought by Ban and a demon in the Fairy King's Forest.

Prologue
Before he became a member of Seven Deadly Sins, Ban, then known as Bandit Ban, wanting to gain immortality, climbed the gigantic tree, in the Fairy King's Forest, where the Fountain of Youth was located. There, he finds Elaine, the guardian of the fountain, who is guarding it in the place of her brother. After staying there for a week and befriending Elaine, he overhears Elaine's wish for him to take her out of the forest, and then offers to do so. She, however, declines, citing her duty to protect the fountain, to which Ban then offers to find her brother, which will relieve her of her duties and allow her to go out. At that moment, a demon suddenly appears and starts burning the forest with his purgatory fire.

Battle
Just as Ban offers to find Elaine's brother, which will allow her to be free, a demon suddenly appears and starts burning the Fairy King's Forest with its purgatory fire. Instructing Elaine to escape with the cup that holds water from the Fountain of Youth, Ban uses his three-section staff to remove the heart of the demon with one hit. However, the demon is not killed, since demons have more than one heart, and then uses its claws to fatally wound both Ban and Elaine, who, in desperation, gives the former the Fountain of Youth's water. Since he didn't have the strength to even drink it due to his wound, Elaine drinks it and makes him drink it through her mouth, thus making him immortal. His wounds now healed due to his newly gained power of immortality, he angrily and quickly kills the demon with his weapon.

Aftermath
After the battle, Elaine dies from her fatal wounds in the now immortal Ban's arms, after telling him to plant the last seed of the forest somewhere, since the forest had been completely burned because of the unnamed demon's purgatory fire. Ban later takes the blame of burning the forest, and killing the guardian or the fountain upon himself, and then starts using a new name, "Undead Ban". The demon's corpse is found by Great Holy Knight, Hendricksen, who discovers the abilities of demon blood and uses it to enhance apprentices' powers as a part of a plot.