Her voice says “Reputation!” but her face says “I killed a hundred puppies for this for this coat.”

Just to clarify, Cinch here is Chrysalis’ counterpart in the Dancerverse, not Sombra’s. Sombra switches with Discord.
Chrysalis <–> Cinch
Discord <–> Sombra

True spooky-scary shadowy umbrum form coming soon-ish.

Queen Platinum is the current leader of the umbrum and her top priority is always the propagation of her species. Wether it’s actually expanding the number of umbrum or ensuring the legends were told about them, she refuses to let her kingdom fade into obscurity.

She has a crown, but she’s not stupid enough to wear it, there’s only one pony in Equestria who has the right to be the Queen, and she sure as heck isn’t going to challenge a physical goddess. She isn’t a slouch in battle, though. She can hold her own.

She’s first encountered masquerading as a unicorn princess from some other continent, there in Equestria to marry Celestia for diplomatic reasons. Moondancer is of course, the only one bothered by this, as her sister* and Celestia were head-over-heels in love with eachother, and would stick to eachother diplomatic marriage or not. Moondancer’s sister, however, is nowhere to be found, but her shield remained. Very peculiar.
*side-note, Knight Watch is a mare now. Realised she doesn’t have to be a dude for a baby to happen. Magic!

Eventually Moondancer catches Platinum casting some sort of mind spell on Celestia, and goes to confront her the next day. She would’ve tried nullifying the spell herself, if she had the chance. Instead, Moondancer is ousted as too overprotective and clingy, where even Flurry just gives her a wordless pat on the back and leaves the room. Platinum stays as if to console her, by sending her deep into the caves under the Empire.

Moondancer wakes up to the face of her then-absent older sister, who explains the situation. Maybe through song, too. She basically had the same suspicions Moondancer did, and also tried confronting Platinum, ending up trapped in the caves. But, together, through the power of a montage, they make it to the upper levels of the cave, escaping just in time to crash the wedding.

Moondancer calls Platinum out on her facade again, using herself and Knight Watch as proof of what she’s done. The crowd breaks out into murmurs, some agreeing, some disagreeing. It’s only when Flurry Heart steps up that they are silenced. With a twirl of her horn, Platinum’s disguise is torn from her, as she is revealed to be, in fact, the Queen of the Umbrum! Platinum tries to buy time for her army with a villainous monologue, which Flurry Heart cuts off like, asap. Moondancer smirks, already seeing the outcome, and it seemed, so did everyone else. Even Platinum was trembling.

A beam-struggle is initiated, and Flurry Heart is predictably winning the battle, but then, most unexpectedly, Platinum gains the upper hand, shooting Flurry Heart out of the air. There is silence, except for a pained gasp from Moondancer as she rushes to her mentor’s side. Then the umbrum break through. The Princess shakes off her injuries as she tells Moondancer to go hold hooves with her friends and do that Crystal Heart thing while she holds off Platinum. She tries doing so, but there are simply too many umbrum swarming in to initiate the friendship nuke safely. Moondancer sees Knight Watch break the spell over Celestia in the cheesiest way possible- with a kiss, and it clicks.

“Knight Watch, use your spell, quickly!”

While both Celestia and Knight Watch were visibly drained from their encounters with Platinum, together (with the power of looove) they’re able to raise a second barrier and send the umbrum flying heck-knows-where, and everything is all sunshine and rainbows from that point on.

Flurry probably starts a campaign to find the umbrum later.

If you’re confused about her name, I sort of headcannon Cinch as being Princess Platinum from the Hearthswarming tale?