In the aftermath of Lily destroying the Resurrection Stone, she became very reflective.
Lily skipped the optional fourth hour of library time after lunch on this Friday and instead spent the hour in her room reflecting on what had happened and who she was.
Lily sat down on her bed just thinking things through.
"I always knew I was very magically powerful witch," mused Lily, "But, to have successfully performed the Killing Curse as a third year? I knew I could do it. But, actually seeing myself do it was a shock to the system."
Lily to reason with and reassure herself.
"It's okay, Lily," thought Lily, "You used it on an object and not on a person. The object in question NEEDED to be destroyed. It's okay, you only did what you had to do. You would never even attempt to use the Killing Curse on a person. And you'd never be able to muster up enough hate for a person to be successful anyway."
This was often a mental dilemma that witches and wizards who used Dark Magic for good faced.
On some level, it always felt wrong to Non Dark witches and wizards to ever use Dark Magic. On the other hand, sometimes it was necessary to use Dark Magic for good. Sometimes, you just had to do what you had to do.
But then, sometimes such witches and wizards would also question who they were and what they could do.
"Could I ever actually become dangerous?" wondered Lily uneasily.
Lily focused her mind, trying to come up with an honest answer to the question.
"Well, " reasoned Lily at one point, "Devon was worried sick about his wand being made of yew when I first met him because yew was also the wood that Lord Voldemort's wand was made of. And look what happened! Devon's yew wand saved Hugo's life!"
Did it all come to choices then, as Dumbledore would say, "it is our choices that show who we truly are, far more than our abilities."
"But then," pushed back Lily, "We're not talking about a wand type here. We're talking about my own magical power as a witch. Can it possibly reach levels of becoming dangerous?"
Lily took a deep breath and cleared her mind and then thought and thought and thought.
"It's still ultimately about choice," realized Lily after a few minutes and a few nanoseconds, "No matter how magically powerful I am or may become, it's up to me how I use that power. I will make the Sacred Promise."
The Sacred Promise was a promise made by a magically powerful witch or wizard to the entire Wizarding World that he or she would NEVER use his or her magical power for evil, to hurt/harm people, or to take advantage of people.
The Sacred Promise was executed by setting one's wand directly against one's heart, then removing the wand again after the seventh heartbeat after the wand was placed there.
One executed, the Sacred Promise constituted a binding magical contract. There could be no backing out once the promise was made.
Any witch or wizard who broke the Sacred Promise to take advantage of another person was magically bound to lose hir or her magic and any witch or wizard who broke the Sacred Promise to commit true evil was magically bound to die instantly.
Lily retrieved her wand from her right hand pocket with her right hand and tapped it directly against her heart, listening intently to her heartbeat.
The nanosecond after her seventh heartbeat, Lily pulled the wand away again and a sudden flash of bright yellow light was visible for a minute and a nanosecond and then disappeared. This yellow light was followed by musical note that sounded like phoenix song and then Lily felt her heart glow with warmth for a minute and a nanosecond before everything was back to normal.
Lily was all in. She had made the Sacred Promise.