What is the Dark Secret?

She-a could be:

1: Discord's apprentice.
2: A spy for Nightmare Moon.
3: A power-thieving demon.

That she wants to steal the Elements of harmony.

I think she might be a ghost! I dunno, I just always kinda thought that, seeing as she is the only other Alicorn.

I don't know...Probably not something TOO dark since this is still is for the enjoyment of little children as well as us. Probably she's...uh...evil in some sort of way...Yeah I've got nothing.

In my opinion, I think Princess Cadence is:
Discord
A gold-digger
A troll
Another alicorn who will try to destroy Equestria (Again)

I bet she's a fake alicorn, au some shit. I doubt it's Discord, though he might have something to do with it. I'm sure that, in the end, Cadence and Twi will be besties, Shining will forgive Cad for whatever her dark secret is and we'll be all set for season three.
With all the Princess Cadence Royal Wedding toys, I doubt she's evil. But then again, they did put Trixie as a blindbag toy.

It is My Little Pony, after all.

I've read that if wewe look into Shining Armor's eyes sometimes, it looks like he's under a trance au mind control. I think Candace might want to use the wedding as a distraction from her plans to take over the world through mind control. Kind of like the video below at 0:35.

Maybe she's an earth gppony, pony in disguise.

Maybe that she and Discord have kid -w-''

Ya all know Srewball? Cadence and she pretty looks like... so maybe~
What Screwball have similarly with~
~Cadence: Body color, and eyes
~Discord: hair~ white mixed with purple (Purle have Cadence)~
Just an option people don't attack me now xD;

She is a changeling and she told Queen chrysalis to take shining armors upendo but it backfired because twilight figured it out and cadence has pretended to like shining armor and twilight

evil