Dragon's Dogma 2

Location: Vernworth
Quest Giver: Brant at The Stardrop Inn at Night
Main Quest: The Silent Throne
Talk to Brant at The Stardrop Inn at night in order to "Tell me of the masquerade", and receive this quest. Some story progress is required for it to appear, but it is NOT missable.

Step 1: The Outfit
You'll need the Courtly Tunic, Courtly Breeches, and Eventide Mask (given upon quest start). There are multiple ways to get this, like sneaking in the Hall near the castle, completing the ornate box quests, but the easiest way is to head to the Comptroller's House near the castle entrance. Just walk in, go upstairs and loot. Once you loot the leg and chest armor, equip all 3 pieces.

Step 2: Entering the Masquerade
The masquerade only happens at night. You can use an inn or a bench to progress time to night. Now, dressed properly, enter the castle walls, go to the right to find the Hall and enter. The achievement unlocks.

Step 3: Secret Entrance
Once inside, go to the hallway that circles the center of the hall. You'll find a secret door here that you can push through to find a passage. Follow the path to a series of cutscenes.

Step 4: Complete the Quest
Upon completion of the cutscenes, return to Brant to finish the questline.

If you like the info drop a like on this short video that also shows how to get it:
There are actually many sets you can get: One shown in video, guest chambers in castle, Sven's Chambers on 2F of castle, Room next to Allard's Chambers, in back room where masquerade ball & if you got and sold all of these, you can farm 300k and buy them in the item shop in the town of the quest.

You can get this during the Main Quest "Stolen Throne" giving by Captain Brant. You will need to either buy the outfit (pretty expensive) at a vendor somewhere or find it. The Video shows you where to get the outfit in a chest and then finishing the quest.