We will explain everything you need to know about the Battle for Katana Kingdom event in Brawl Stars.

The Battle for Katana Kingdom is a four-week-long Brawl Stars event that will dive into a story where Kenji tries out a new sushi recipe that creates a whole world of chaos.
There are four Chapters in this event, with each Chapter lasting for a week and coming one after the other. Each week (or Chapter) will see the arrival of awesome rewards, game modes, and exclusive items.
Held between May 1 and 30, this limited-time event promises nothing but all-out action and incredible rewards for both you and the rest of the community.
All Chapters in the Battle for Katana Kingdom Event
We'll break down what each Chapter will bring to Brawl Stars in the following sections.
Chapter 1: Way of the Wasabi
Chapter 1 begins on May 1. It will see the introduction of Sushi Rolls, Wasabi Powers, and the Samurai Smash game mode. We'll break down what they are below.

First off, Sushi Rolls are blind boxes with a similar mechanic to that of Starr Drops. You can swipe each Rolls, for up to four times, for a chance to split them into additional Sushi Rolls. They come in four rarities: Rare (1 Sushi Roll), Super Rare (2 Sushi Rolls), Epic (4 Sushi Rolls), and Mythic (8 Sushi Rolls). Each Roll grants you one reward.
You can earn rewards such as Coins, Hypercharges, Power Points, Brawlers, and other event-exclusive cosmetics.

Secondly, Wasabi Powers are Brawler-specific modifiers that give special abilities. There are 60 of them in total, with them being divided into four rarities. There are 20 Hot Wasabi, 15 Extra Hot Wasabi, 15 Crazy Hot Wasabi, and 10 Inferno Wasabi. You can unlock Wasabi Powers from Sushi Rolls.
Finally, Samurai Smash is a PvPvE game mode where you engage in 3v3 or 5v5 battles. You'll also face Robot Samurai, who will attack both teams with their unique attack styles. The objective is to collect as many Daruma Dolls as possible, which are scattered around the map, before the timer ends.

The first team to collect 50 Daruma Dolls in 3v3 wins, while the first team in 5v5 to collect 100 Dolls wins.
Chapter 2: Rise of the Futomaki Five
Chapter 2 begins on May 8. It will introduce the Mega Buffet Club event as well as a Mastery Madness Weekend.
The Mega Buffet Club event runs similarly to the format you're probably familiar with in the Mega Pig event. Players get four tickets each day (which can be stacked) to spend on playing Mega Buffet game modes, which have Wasabi Power and other modifiers active.

Each match win contributes one point to the Mega Buffet meter. All Clubmates will earn one Sushi Roll after reaching a milestone, which can be opened immediately. But keep in mind that you'll need to contribute at least three wins to earn these rewards.
Besides this, the Mastery Madness Weekend will be available between May 10 and 11. You can earn additional Mastery Points from winning matches during this time.
Chapter 3: The Taste of Vengeance
Chapter 3 begins on May 15. It will introduce the Oni Hunt game mode and a Double XP Weekend.

Oni Hunt is a PvE game mode where three Brawlers will take on a massive, Hypercharged demon Kenji. Winning matches can reward you with Coins and Sushi Rolls. Kenji will have various attack styles, so be wary of them and the damage they can inflict. You will lose the match once all teammates are eliminated.
After the game mode is made available, you can earn seven days of rewards by beating Kenji's five stages of difficulty each day.
And, of course, a Double XP Weekend will be held between May 17 and 18. You can earn additional XP for the season's Brawl Pass during this time.
Chapter 4: The Final Cut
Chapter 4 begins on May 22. It will introduce Daily Contests and a x2 Starr Drop event.

In Daily Contests, you'll need to pick from one of three game modes/maps each day. You'll then be placed into a 10-player leaderboard with others who'll also compete in the same mode/map as the one you've chosen.
The goal is to get as high a rank as possible. The higher your rank, the more Sushi Rolls you'll be rewarded with. This extends to the top eight. Unfortunately, the bottom two players will not receive any rewards.
You'll be given four Tickets each day (which don't stack over days) to participate in the Daily Contest, so be sure to use all of them.
The x2 Starr Drop event will enable you to get an additional Starr Drop for getting your first, fourth, and eighth win in your Daily Win Milestones.