As Meliodas was the second person after Elaine to acknowledge him, Ban joined the order.
After the Sins were framed, he let himself be captured and imprisoned in Baste Dungeon, only to escape years later when he discovered that Meliodas was still alive.
For his inaction to stop his friend from doing so all those years, King was sentenced to an imprisonment of over 1000 years.
In the midst of it though, he was recruited into the Seven Deadly Sins.
While Ban didn't defend himself for the crime that he was accused of, he however at some point in time actually revived the forest by planting the seed entrusted to him by Elaine.
For this, he was deemed the new Fairy King by the Sacred Tree and accepted as such by most of its inhabitants much to his reluctance.
It was only when he reunited with his friend again did King learn that his old friend was massacring countless humans over the centuries, an act which was practically a declaration of war between the Human and Fairy Clan.
In a sheer act of sacrifice, Ban was saved by Elaine when she gave him a drink from the fountain instead of saving herself.
Having become immortal, Ban took care of the Demon but the damage to the forest was done.
For the next 500 years, they spent their lives together and fell in love with one another.
Due to circumstances involving King however, he had to erase her memories and leave her.