Ban vs. Red Demon

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

Prologue
Ban, desiring immortality, goes to the Fountain of Youth in the Fairy King's Forest, where 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 offers to take her out of the forest, when he hears her desire to go out, which she declines, citing her duty to protect the fountain. He then offers to find her brother, so that she can be free and go out of the forest. At that moment, a demon suddenly appears and starts burning the forest with his purgatory fire.

Battle
Just as Ban tells Elaine that he will find her brother, so that she can be free, a demon from the Demon Clan suddenly appears and starts burning the Fairy King Forest with its purgatory fire. Telling Elaine to get the cup and escape, Ban uses his three-section staff to remove the heart of the demon with one hit. However, the demon turns out to still be alive, since demons have more than one heart. It then uses its claws to fatally wound both Ban and Elaine, who, in desperation, gives the former the cup which has the water from the Fountain of Youth. Since he didn't have the strength to even drink the water 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".