Challenge

Challenge mode is a new event mode which will be added in Sizzle Season 2023. Officially a replacement for League Battle, Challenges are limited-time competitions of standard modes with unique in-match conditions. They can be played solo or with a team, and feature a two-hour ranked play window similar to League Battle, but unlike League Battle's constant rotations, the windows only occur at a few specific times during a season. The monthly event battles that appeared in League Battle mode were confirmed to return as Challenges.

Unreleased Challenges
Seven types of Challenges with unique conditions exist in the game's files but not yet have been revealed: "Foggy Notion", "Extreme Jump Battle", "Duel of Dynamic Duos", "The Sheldon Sampler Challenge", "Zip-to-It Zipcaster Challenge", and "Art Battle for the Ages".

Quotes
"Ballistics aside, we have an update about the League Battle feature from Splatoon 2. A similar feature is coming in Sizzle Season with a new name - Challenges! These are an exciting new way to battle with special rules for a limited time, and you can join solo or with friends."

- @SplatoonNA on Twitter

"A new mode - Challenge - is coming to #Splatoon3 in Sizzle Season 2023! Look forward to monthly challenges and events with special rules. Test your Turf War talent on a brand new stage in the first event, taking place 03/06!"

- @NintendoEurope on Twitter