What spell does Harry discover written in the Half-Blood Prince's potions textbook in Book 6, and who invented it?
Sectumsempra, Severus Snape.
<s>[INST] <<SYS>> Answer as briefly as you can to the following question. You are required to give a one-shot answer: <</SYS>> What spell does Harry discover written in the Half-Blood Prince's potions textbook in Book 6, and who invented it? [/INST] Sectumsempra, Severus Snape. </s>