So in theory, Chara fell into the underground, got adopted by the royal family, found a poisoness flower and killed themself to merge with Asriel to kill all humans but failed when Asriel resisted causing them both to die.

Then they most likely used Asriel by playing to his nice side making a last wish that could only be accomplished by them merging souls. The tapes in the true lab heavily imply that chara poisoned themself with buttercup flowers that they learned were poisoness after accidently using them in a pie for Asgore. He also says Chara, for unknown reasons, hates humanity. Asriel says when you talk to him after walking to the very first room in the ending sequence that the control was split between them when he absorbed Chara's soul and Chara's soul wanted to kill the humans but he resisted (causing him to die of wounds). You will find yourself at the true lab where you learn by watching the tapes that Chara had a dark plot to kill themself, possess Asriel and use their immense power to destroy Humanity. At first, it seems like a tragic tale for the human but there is a dark twist. However, this kills Asriel in a tragic misunderstanding by the humans. Asriel does this by absorbing Chara's soul and crossing the barrier. This causes Asriel to want to fullfill the humans last wish to see the flowers from their village. 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.

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