After the king and queen take in Chara they fall deathly ill and die. Later in the game you will be treated to the story of Asriel and Chara. The injured Chara calls for help then Asriel saves them. After an unexplained amount of years, Chara falls into the underground. This is where Chara (the first human, the one you named) comes in. Many years later after being trapped underground, Queen Toriel and King Asgore have a child named Asriel. The barrier is a magical seal that can only be broken by 7 human souls or exited by a human and a monster soul fusion and it can be entered by anyone. Undertale starts off by telling us the story of the monsters war against the humans that ended in the monsters being trapped underground by the barrier.
