To Ravenholdt

Make sure you talk to Mostrasz in order to fly over to Ravenholdt, rather than flying yourself.

Apparently some Rogues (mine) are less crafty than others...

I went and started the quest chain for the Fangs of the Father Legendary during the PTR and copied the transcript of the RP that Mostrasz gives while you're flying from Twilight Highlands to Ravenholdt.

SPOILERS!

Mostrasz says: Let's move out, friend! There's no time to lose.
Mostrasz says: Corastrasza is understandably upset. Under her care was the one black dragon egg that we believe was free of the taint of Deathwing's corruption...
Mostrasz says: ...and it was stolen away from us by thieves in the night!
Mostrasz says: You've found for us our only lead. The rogues from Ravenholdt are behind this.
Mostrasz says: Ravenholdt? It just doesn't make sense! A mongrel band of petty thieves. How did they even KNOW about the egg?
Mostrasz says: We'll get to the bottom of this. Someone with your skills is exactly what we need.
Mostrasz says: What's the expression? "To catch a thief," they say?
Mostrasz says: Look at this devastation. These foothills have never recovered from the campaigns of the second and third wars.
Mostrasz says: Ruins and debris litter the landscape... and I consider the thieves and assassins of Ravenholdt to be part of that debris.
Mostrasz says: Without a state... without a cause... what drives them, do you wonder?
Mostrasz says: And what possessed them to strike out against the red dragonflight? Madness. We must learn more!
Mostrasz says: We're almost to Ravenholdt manor.
Mostrasz says: If they have the egg, it's too dangerous to risk an all-out assault. You'll need to sneak in there, find out what they're up to... And retrieve the egg at any cost.
Mostrasz says: Quiet now! We'll form up a plan within this cave...

You're a rogue. Loyalty is a transitory affair...