This one is quite dark. Trigger warnings for child murder and patricide. The prompt is:
The wind and rain had scoured the stones clean long ago. All that was left of the sacrifice, perhaps only a few molecules, was consumed by the lichen and moss centuries past. It gave Joshua some comfort, at least, to hope that the blood of his sister may have nourished more than the insatiable ambition of the wizard Salazar—his father. Jocelyn was her name, his twin. Josh and Joss their mother called them, always together, always in the same breath. They shared everything, including the powerful magic that flowed through their veins. Their father, though, saw Joss simply as a source of power that let him use his son, not as the object of the blood magic, but instead as his unwitting apprentice. "Mix these herbs into your mother’s and your sister's evening tea," he ordered Joshua. "And when they are asleep, bring your sister to our sacred clearing in the wood." Having obeyed his father, Joshua watched innocently as the old wizard chanted the ancient words of the forbidden spell while he laid Joss across the altar. But when Salazar slit her throat and her blood drenched the stones and all the ground around them, Joshua realized what evil plan he had helped his father bring to fruition. Horrified, he drew his own knife and drove it into his father's heart just as the wizard uttered the final words of the incantation. With his last bit of strength, Salazar grabbed Josh and pulled him down into the swirling and smoking mixture of their magical lifeblood, a bath that granted his son the power and immortality he had meant to claim for himself. As he had on this day for centuries, Joshua added a drop of his own blood, and more than a few tears, to the lichens' feast.