Once upon a time, unknown centuries in the past, monsters and humans dwelled together peacefully on Earth. However, one day, for reasons unknown, war broke out between the two races. The fight was long and hard, but at the end of it, the humans came out victorious. They locked the monsters underground with a magical spell. Many years later, a human named Chara falls into the Underground and is discovered by Asriel Dreemurr, the only son of the king and queen, Asgore and Toriel Dreemurr. Asriel brings Chara to his home, where she is adopted by the Dreemurr family despite being a human and lives with them.
Unfortunately, this doesn’t last.
Chara, learning that seven human Souls and one monster Soul were needed to break the Barrier set in place by the spell, hatches a plan with a reluctant Asriel to collect those human Souls. Poisoning herself, she dies and Asriel absorbs her Soul, becoming an incredibly powerful monster-fusion. Crossing through the Barrier (since only a single human and monster Soul are needed to cross through) with Chara’s body in his arms, he heads straight to Chara’s old home and town. As he lays her body down in the bed of golden flowers that she’d desired to see one last time, screaming catches his attention. He’d been spotted by the human inhabitants of the town, who thought that Asriel had killed Chara. Immediately the humans attack the monster prince. Yet, despite having the power and motivation to destroy them all, Asriel couldn’t bear the thought of killing another, so, taking up Chara’s body again, he turned and left the way he’d come.
Fatally wounded, he barely made it back through the Barrier before collapsing. His dust spread on the flowers growing on the floor of the throne room.
The monster civilization was in turmoil. In a mere few hours, the Dreemurrs had lost both of their children. Stricken with grief and enraged, King Asgore once again declared war on humankind. Any human that fell into the Underground would be killed and their Soul would be used in the breaking of the Barrier.
Many more years pass. Six human children fall into the Underground and are killed, their Souls stored away in special containers located near the Barrier. Meanwhile, Toriel, disgusted by her husband’s actions, casts aside her crown and exiles herself to the Ruins (the first area you pass through in the game), where she attempts to shelter and protect the humans that fall but always inevitably failing. Then, one day, a child, wearing a purple-striped blue shirt with a Soul as red as Chara’s had been, falls into the Underground.
How could a kid understand?