If you're currently pursuing The Stolen Throne quest in Dragon's Dogma 2, chances are you may have tried to walk into a masquerade ball only for some guards to get angry, kick the crap out of you, and throw you in gaol. This is a slightly unusual quest; you require a specific outfit—formal raiment—to enter the party, but it doesn't list acquiring it as an actual step. The only hints you get are from Captain Brant and in the quest's description. 

When Brant asks you to infiltrate the masquerade to locate the false Sovran, he also gives you the Eventide Mask that you'll need to wear. Here I'll cover how to acquire your formal raiment for the masquerade ball, plus what you actually need to do once inside, since it's pretty unclear in the game.

How to get formal raiment

Though not an official quest step, you need to acquire fancy clothes if you want to gain access to the masquerade at Vernworth Hall in the castle. If you try to get in without wearing them or the Eventide Mask that Brant gave you, the guards will turn hostile and attack. If this has already happened, simply sprint out of the castle grounds and they should turn back to normal. 

Now, the outfit you're looking for consists of the Courtly Tunic and the Courtly Breeches, and there are two ways you can get these:

• Grab them for free in Vernworth Castle from either the chest in Sven’s Chambers or the chest in the Guest Chambers on the second floor. Make sure to visit the palace in the daytime so you can access the interior and get to the upper floors where these rooms are.
• Purchase them for a hefty sum of gold at Philbert’s Sundries in the Merchant Quarter.

Honestly, there's no point doing the second option unless you enjoy wasting money. Once you've got your formal raiment and have equipped both it and your mask, head to the castle at night, and you'll be allowed to enter Vernworth Hall without the guards getting angry.

How to complete The Stolen Throne

Now you've gained access to the masquerade, you need to find and approach the false Sovran, though this is a little more complicated than you'd expect. You can walk around the party chatting to random nobles, but besides one who swears he recognises you, the Sovran is nowhere to be seen. 

What you need to do is find the secret door at the back of Vernworth Hall. This is easy to spot since it's a bricked up stone doorway in between two smaller rooms. Just walk into the doorway and it'll swing open revealing a secret passage leading to the Rose Chateau Bordelrie and a cutscene that continues the quest.