Many nobles tried to prevent the Regent's rise to power, but there was little to nothing they could do in the end. Flurry Heart was the next Princess, and the Regent would watch over her.
The Loyalists were born almost immediately after, hidden in the shadows. Coming together, they decided this couldn't happen, and that the Regent was simply using the young Princess to satisfy his own hunger for power. Within that night, Blueblood disappeared — and just in time, as the Loyalists began hearing rumors of others who might have claim to the throne being assassinated in the dead of night.
Within the following months, taxes climbed ever higher, chafing smaller towns and destroying some. The Loyalists grew in turn, as many sought to help those who would end the oppression. The Regent would find a growing frequency of attacks and raids on his soldiers, the occasional assassination of allies, and once his entire drawr of socks disappearing without cause. Growing ever more paranoid, he increased the guard presence further.
Still, not everypony liked the Loyalists. Some felt they were making things worse than they needed to be, and yet others felt that their goals were no more innocent than the Regent's. Still, none would argue they helped give many ponies the bravery they needed to stand up and fight.
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The Loyalists are a conspiracy whose overarching goal is to put Prince Blueblood on the throne. They believe that Blueblood has the rightful blood claim as well as a greater deal of experience in politics, and accuse the Regent of usurping the throne by taking control of Princess Flurry Heart. They come from many backgrounds, though the ponies in charge are often from backgrounds of wealth, military service, and trade.
The Loyalists are the longest standing enemy of the Regency, and their tactics tend to be more subtle than most. Rather than waging an all out war, they often employ assassins, extortion, misinformation, and secret spy networks. Their foot soldiers tend to be regular ponies who have grown tired of the Regent and his oppressive ways.
The Loyalists have no primary headquarters, and they span the whole of Equestria.
Among their leaders are Rarity, Pinkie Pie, and Fancy Pants. While none of them really like Blueblood, each of them agree he is the better choice than the Regent, though some ponder if their goal is simply to install Blueblood as a figurehead for them to control behind the scenes.
Rarity joined the Loyalists at the request of Fancy Pants, who knew Blueblood needed all the help he could get. While she feared it would put her family on the line, she knew the oppressive regime of the Regent was too horrible to bear, so she joined in. She quickly became one of the top members of the conspiracy, and indeed, it almost resulted in her assassination.
Pinkie joined the Loyalists after an assassination attempt that was meant to kill Rarity killed her sister, Maud Pie. Rarity had managed to barely survive, though she had a scar on the right side of her face as a result. It was only later that Pinkie learned that Maud herself was a member of the Loyalists and had been there to warn Rarity of the attempt. Angry, Pinkie sided with Rarity in the hopes of avenging her sister.
Art (C)
MLP:FiM (C) Hasbro/Lauren Faust