In the Dark Fist Tournament, you have to "fight" the Terrible Four.
The first underground location in Honkai: Star Rail that we will visit is Kamnegrad, where you can meet a lot of new characters. In this region, the Pathfinders will find a lot of interesting tasks and secret side quests.

One of these side missions is a rock-paper-scissors game with a group of kids, but this quest lacks any clues, so you'll have to deal with the stages of its passage on your own. This guide will help you get into the Dark Fist tournament and successfully complete it.
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How to get to the tournament "Dark Fist" (secret quest)

In order to trigger this side mission, you will first need to find an invite to the Dark Fist Tournament. This item is in the Underdark Fight Club.
After opening the Kamnegrad location, move to the Fight Club spatial anchor and move west to find this institution. If you didn't have time to open all the teleportation points in this region, then go to the spatial anchor "Natasha's Clinic" and move north from it. This path will lead you straight to the fight club. Once inside, turn left and approach the crate near the table. Click on the interact button with the box to pick up an invitation to the Dark Fist Tournament. This is an ordinary black envelope, on the back it says in large crooked letters: “Invitation”. When you open the envelope, you will see a painted sheet of paper with the inscription: "I see that you have been chosen ..."
How to use the Dark Fist Tournament Invitation in Honkai: Star Rail

The rest of the quest can also be completed in Kamnegrad. You need to exit the fight club and turn left, where you will see a Mystery Child near the stairs. Give him an invitation to take part in the Dark Fist Tournament. It turns out that this tournament is a game of rock-paper-scissors, in which the Pathfinder will have to fight the Fearsome Four. According to the Mysterious Child, if you manage to defeat all opponents in one round, you will receive an incredible reward.

In fact, it is enough to defeat at least two opponents out of three. During the game, it does not matter at all which piece you will show (rock, scissors or paper), as the results will always be the same: the first and third members of the Terrible Four will lose, and the second opponent will win.
How to get achievement 12+ in Honkai: Star Rail

Completing this mini-quest will unlock the 12+ achievement, as well as 20 shields and the recipe for Gauntlets of Wrath, a consumable item that increases the attack power of all allies by 5% and grants an additional 170 attack power in the next fight.