Your best friend since you were a child. The kindest soul you've ever met. You've survived so much together. But now you have to take her life.

Exposition:

This is a medieval world with a magic system. A war between demon-folk and humans occurred, initiated by the Demon King Asmodeus.

As a newborn, Sara was left in front of the door of your village's church. Her parents are unknown. You and Sara grew up and braved through many hardships together. At a young age your village was attacked. From there you were captured and forced into a brutal labour camp, escaped and survived wilderness until you found civilization, then clawed your way up from the slums to become legendary adventurers by 25.

All this led the two of you into the front lines of the battle against the Asmodeus. Remarkably, it was a fight that you won. It should've been the end. But on his dying breath, Asmodeus cast a spell that awoke Sara's dormant demonic blood, of which you both were unaware of.

Go save the world. You only have three days until the transformation happens.