Undertale spoilers
@kara_dreamer@witches.town by this, resulting in Asriel innocently joining with Chara's Soul to have the power to go past the barrier and carry out Chara's dying wish, to return to the humans. It's stated by Asriel though that HE was the one who held them back from killing all the humans above ground like Chara wanted, so he let himself and Chara die rather than hurt them all, but YEARS without a soul broke him and only once he had a soul again did he realise what he'd done.