To celebrate the end of the year, owners of the Three Houses expansion pass can download the “Sothis Regalia” costume, which lets the protagonists dress up as Sothis.

By the way, the Japanese website explicitly states that this will be the final costume DLC for the game.

Looking back at the upcoming DLC info from the Famitsu guidebook, you can see that this costume was originally part of the 4th and final wave of DLC. We translated it as “new costume(s) for male Byleth, female Byleth and Sothis”.

However, the wording was rather unclear; it was probably intended to be read as “new Sothis costumes for male Byleth and female Byleth”. Indeed, I’m sure many fans were wondering how they’d implement a costume for Sothis, who’s a NPC.