In the Interest of Vengeance

First Year's practice simple spells for weeks before mastering them, but Harry Potter managed to whip out Sectumsempra on his first go? Highly unlikely. An exploration of how he might've geared up for that fateful altercation with Draco, and how it might *not* have gone unnoticed by a certain Professor who knew it all too well. Trigger warning: implied violence, blood