Isekai Quartet

Isekai Quartet (異世界かるてっと, Isekai Karutetto) is a Japanese anime series that serves as a chibi-style crossover between the series KonoSuba, Overlord, Re:Zero − Starting Life in Another World, and Saga of Tanya the Evil, all published by Kadokawa Corporation. The series aired its first season from April to June 2019. A second season aired from January to March 2020, including additional characters from The Rising of the Shield Hero and Cautious Hero: The Hero Is Overpowered but Overly Cautious. A third season has been announced.

Synopsis
One day, a magic button suddenly appears. The protagonists from KonoSuba, Overlord, Re:Zero − Starting Life in Another World, and Saga of Tanya the Evil all press the button occasionally, unintentionally and move to a parallel world — another brand new Isekai — where a new story featuring high school life begins. Soon, they are joined by protagonists from The Rising of the Shield Hero and Cautious Hero: The Hero Is Overpowered but Overly Cautious. With no way of escaping, the characters must lead a normal school life and make acquaintances with the others. One thing is certain: the classroom full of fan-favourite eccentric personalities never gets boring!

Theme Songs

 * Opening Season 1 : Isekai Quartet (song)
 * Ending Season 1 : Isekai Girls Talk
 * Opening Season 2 : Isekai Showtime
 * Ending Season 2 : Defective! Isekai Theater