During one night in a human house, two siblings were arguing about the fate of monsterkind.
The BRAVERY wizard (Agate Lightvale) wanted to keep the barrier because it was a good measure in order to keep peace between the two races. She wanted to follow the king's law.
The DETERMINATION wizard (Copper Lightvale) wanted to break the barrier.
There was someone else implied in those incidents (Amber Lightvale) who will be an important character later.
We do not know how did the first battle took place. However, the first battle between Agate and Copper was organized. The humans were watching them fighting (including Amber who was sitting next to a human guard) in order to decide what the fates of monster kind will be. At the end of the battle, Copper won and Agate was humiliated because of her defeat. She lost her trait and fleed.
As for the second battle we do not really know how the battle took place (However in the animation, it was seen that the battle was during night time in their home). It was set when Agate somehow "recovered" her trait and returned at home. She then forced her brother to fight her first as she pointed her spear at him like a death threat, Copper denies the offer then Agate beomes furious and insane. And because of this, she killed Amber, this caused Copper to lose his trait and that led Agate to victory by killing Copper when he no longer had his trait.
Agate then carried her sister's corpse and performed the "Bête Noire" spell on it with her soul.
|Pre-Glitchtale Battles||Chara vs Asriel ◆ The Great Human and Monster Wars ◆ Agate Lightvale vs Copper Lightvale ◆ Integrity vs Asgore ◆ Flowey vs Sans ◆|
|Season One Battles||Frisk & Chara vs Sans ◆ Gaster vs Frisk & Sans ◆ Chara vs Sans & Flowey ◆ Chara vs Omega Flowey, Sans, Frisk & Gaster ◆ Asriel vs Chara◆|
|Season Two Battles||The AMD's trap ◆ Battle on The City ◆ Gaster vs Bete Noire ◆ Undyne vs Bete Noire ◆ Frisk vs Bete Noire ◆ Asriel & Bete Noire vs Frisk, Toriel & Asgore ◆|