Blood Magic Lore

Sacrifice of the Few
Initially, Blood Mages had the ability to use their own blood in addition to the blood of others. A set of particularly powerful Mage twins were experimenting with spells one day, and one of the siblings, the female, had lost too much blood during the ritual. By the time her twin, a male, had noticed, it was too late. His heart was beating too fast, all he could do was watch his twin sister bleed to death on the floor in front of him. He could not calm himself down in time.

Instead, he got himself even more worked up, completing the spell that claimed his sisters life, with a few alterations. It was infinitely larger than the brother anticipated, and seemed to affect the whole planet. None of the blood Mages could seem to use their magic with their own blood. The brother eventually committed suicide out of grief of his sisters death.