The next Tetris 99 event is a crossover with one of Nintendo's most highly-acclaimed titles of 2019, as players will be competing on a stage based on Fire Emblem: Three Houses.
Tetris 99 usually only allows players to use a single stage, but it has been possible to unlock more by competing in online events. The previous stage themes that have been used in Tetris 99 events include a retro-themed Tetris stage based on the original Game Boy title (including the classic music), while the most recent stage allowed players to win a theme based on Splatoon 2.
The next Tetris 99 event will be a crossover with Fire Emblem: Three Houses, which has been a surprise hit for the Nintendo Switch and has been well-received by both critics and longtime fans of the series. It's unclear exactly what the stage will look like or what music will be used, as the event has only been promoted with posts on Nintendo's social media accounts. It's possible that the background will be a piece of key art from the game, with the main theme used as its music.
The Fire Emblem: Three Houses theme will be used in all online Tetris 99 matches throughout the duration of the contest, but the player needs to compete in games in order to be able to keep the theme after the event has ended. In order to do this, the player needs to earn one hundred points over the course of the event. The player will earn points depending on their placement in matches, which means that skilled players should be able to unlock the Fire Emblem: Three Houses theme rather quickly. The less-skilled players will likely take longer to unlock the theme, but it's still possible, as even placing last will net them points.
It seems that the crossover events are becoming a regular occurrence for Tetris 99, which means that there might be other crossovers with different Nintendo franchises in the future. The Legend of Zelda: Link's Awakening and Pokémon Sword & Shield aren't that far off and they would be excellent candidates for Tetris 99 crossovers.
The Fire Emblem: Three Houses crossover event for Tetris 99 will run from August 23 to August 27.