“Get. Her. NOW!!
 
“It’s all on me! I’m the one Tempest wants!”
 
 
OH BOI IT’S HEADCANON \ CONSPIRACY THEORY TIME!!
 
Those lines that I’m referencing ^^ are from the MLP Movie. Has for how they fit in the actual story, we don’t know - what we do know is that Tempest Shadow and the Storm King are clearly here for something. But for what?
 
My theory? They want Twilight Sparkle. Not Twilight herself, but her Alicornhood.
 
Tempest was once Celestia’s student way back when [ before Sunset or Starlight ] - she didn’t have evil intent and was pretty successful like Twilight. She was almost ready to ascend to Alicornhood. But then something happened.  
Tempest got involved in an accident - a pretty brutal one that took out her horn deep into the quick. This resulted in being unable to use magic properly, not being able to function without an entire horn to call upon and rendering non-basic spells useless. Celestia had to drop Tempest out of The Magic School For Gifted Unicorns, recognising that her disability would hinder her studies and hold her back from ever earning her wings, let alone use proper magic.
 
This naturally made Tempest bitter and pissed off that such a tragic accident - that was no fault of her own - destroyed years of hard work and determination. Unlike most ponies, she didn’t spiral down into depression, instead becoming angry, impulsive and cold. She had a tendency to lash out and become physically violent towards others around her, to the point where her parents [yes, she has parents] had to give her away to a family friend of theirs who managed troubled ponies. This said pony suggested working with the Storm King [who was opening up new spots in his military] and using tactical army procedures as an outlet for her issues.
 
Tempest loved being able to beat the shit out of anyone and everyone and get payed for it. She realised that she could physically handle her targets through combat rather than use magic, which was needed as a solider. If she felt particularly angry that day, she could rip an opponent a new one and nobody would bat an eye. The Storm King particularly loved her and decided to make her commander.
 
Despite having all this power, Tempest still felt like she was missing something - which she was. Not being able to become an alicorn still gutted her - even years after the accident occurred, she’d still lay in bed feeling guilt for not feeling like a ‘proper’ unicorn. This ‘missing piece’ clicked in when she heard of the new alicorn that was ruling over Equestria. Tempest’s buried fury rose back up again like a volcano and she became enraged. She knew what she had to do now.
 
She’d take Twilight’s horn and wings and rightfully claim them as hers, dragging her entire army over to Canterlot to siege the city that could’ve been ruled by her.
 
In Tempest’s eyes, Twilight Sparkle doesn’t deserve Alicornhood.
 
She does.