Willpower (an Undertale fanfic.)

In the undertale universe a human fell down into the underground (AKA the monsters' home) this human is known as Frisk. She (Canonically she is gender neutral but for the sake of the fanfic they will be known as a she) befriended the entire underground by freeing them from a seemingly eternal spell that trapped them down there for many years. Though to get there Frisk had to sacrifice a friend (Asriel) and from that day on frisk just couldn't get the fact that she couldn't save someone out of her head. And she will soon find out that she'll have to sacrifice a lot more.