And Then There Were None

Story
And Then There Were None is a murder mystery about 10 people convicted of murder are invited to an island for a "vacation". It was Agatha Christie's best selling book, making it way up as one of the best selling books of all time.

Reference
One of Flandre Scarlet's spellcards and her theme are based off of this book. Her 12th spellcard is called "And Then Wlii There Be None". It relates to the poem (as its called in the story) Ten Little Indians (note: Ten Little Indians and Ten Little Soldiers are the same thing, just different characters). 

Flandre's boss theme, U.N. Owen was Her?, is also referenced. The person who apparently owned the house were both named U.N. Owen. Notice how it similer to "unknown". Whoever was U.N. Owen is the killer. So her theme song "U.N. Owen Was Her?" might give you a clue on who the murderer is...or not.

Spoiler down below

Now after you beat Flandre with Marisa, they start talking about the poem. But there is one part in particular that Marisa says: "Hey, don't you know the real song? She got married and then there were none." Actually, one of the characters, named Vera, thought that was the last part of the poem after killing Philip Lombard. She is heartbroken after her love left her. And her losing most of her sanity after she killed his nephew might have something to do with it..