@Etherchanter  

 
Well we officially have Daybreaker now so Heartbreaker would fit right in.

Cadence and Shining Armor seemed like they had the perfect relationship, being the Prince and Princess of Love, but nothing could be further from the truth… Being that Cadence usually had to take care of royal business in the Crystal Empire, and Shining Armor still had his duties as captain of the guard, the two rarely had time for each other anymore… And being that Shining Armor was still any normal stallion, that meant he was getting bored… And it turned out there was another princess who he knew was just as bored as he was…
 
Cadence had returned home early from a business trip to Saddle Arabia, and was hoping to spend the free time she had with her darling husband… When she reached her bedroom, she found Shining Armor wrapped in the tender and loving embrace of… PRINCESS LUNA!!!
 
Consumed by anger and jealous fury, Cadence summoned all the alicorn magic she could muster and hurled it at Luna in a blast that could shatter stone! The Princess of the Night cast a reflection spell in self defense, sending Cadence’s attack flying back straight into her! Cadence was left sprawled on the ground, a sobbing broken wreck, glaring at her sister-princess and her soon-to-be-former husband with eyes filled with pure rage…
 
“Get… out… Both of you! Get… OUT!!! I HATE YOU!!!”
 
Luna and Shining could do nothing but flee the palace and go their separate ways, but unknown to them, Luna’s counter-spell had done more damage than she thought… Her spell had laced Cadence’s attack with some of Luna’s dark magic, the same magic that had turned Luna into Nightmare Moon over 1000 years ago! And now that magic was feeding on Cadence’s hatred and sadness, growing inside her and filling her with dark thoughts and powers… Her beautiful pink coat turned a shade of dark magenta, and her perfect royal mane became a translucent mass of seductive energy… Her sobbing turned into chucking, which evolved into wicked laughter as Equestria’s new monster was born…
 
“Soon, all ponies who think there is such a thing as ‘true love’ shall forever know MY pain… I will show them all… the pain of… HEARTBREAK!!!”

How about Nightmare Mourn?  
Only if this is if her heart is broken in anyway.

@Sparrow9612  
I like that.

Perhaps Nightmare Heart? The Princess of Hate.

I’m thinking rather than Heartbreaker, how about just Heartbreak?

I know Kingdom Hearts kinda stole the term..
 
But what about Heartless?

@BronyHeresy  
what about the name Decadance for pony Slaanesh

Heart Breaker

I kinda like Heartbreaker, myself…

“ALL SHALL LOVE ME AND DESPAIR!”

The lust will last forever!

I call her Ansem

Interesting concept.

Yandere Cadance?

Honestly, I’d expected a Nightmare-ified Cadance to be more like Chrysalis, really.

Nah, I agree with the clop filled thing, but nore along the lines of love taken to its negative logical conclusion. Basically, Slaanesh.

@Random:
 
Loveless sounds good.
 
inb4 Genesis of FFVII:Crisis Core

If she wants to create a loveless world, there could possibly be a Mane Six/Chrysalis team up.

i kinda like Loveless.

I would call her Queen Heartless, personally.

@Dawn Blast
 
Y’know, that is sounding like a much nicer theory. :D

Heartbreaker seems to make sense.

I like Heartbreaker honestly.

@sixfour215
 
Nah. I can sum up her evil plot in this sentence…
 
“LET THE CLOP FILLED PARADISE BEGIN!”