This can be unlocked by evolving the Pentagram weapon. To do this the weapon itself must be level 8 and you need to have the Crown equipped. Then find a treasure chest and the Pentagram will evolve into the Gorgeous Moon.
Pentagram is unlocked by surviving for 20 minutes as any character and the Crown is unlocked when you reach level 10 with any character, so you likely already have access to them.
The Pentagram is the starting weapon for the Christine Devian character so using her will give you a leg up but it's not necessary as it can be equipped on any character that it drops for.