Isekai Quartet

Isekai Quartet (異世界かるてっと) is an anime crossover spin-off that first aired on April 9, 2019. It is a crossover anime series that consists of characters from four major light novel works that boast a cumulative total of over 16 million copies in the total series, and boasting a total sales of more than 500,000 Blu-ray Disc & DVD series: Overlord, Kono Subarashii Sekai ni Shukufuku wo! (この素晴らしい世界に祝福を！), Re:Zero kara Hajimeru Isekai Seikatsu (ゼロから始める異世界生活), and Youjo Senki (幼女戦記).

The series is produced by Kadokawa and the studio in charge of animation is Studio PuYUKAI. Asini Minoru is the director/screenwriter; Takahara Minoru is the character designer and general director. The latest news and information are posted on the series official site. The opening theme for this series is Isekai Quartet and the ending theme is Isekai Girls Talk.

A Season 2, titled Isekai Quartet 2, was announced on October 5, 2019, and was released on January 14, 2020. The season introduces new characters from a fifth major series: Tate no Yūsha no Nariagari. The opening theme for this series is Isekai Showtime and the ending theme is Ponkotsu! Isekai Theater.

Summary
A mysterious switch appeared one day. When pressing it, they were sent to a different alternative world!! There are also characters from other alternative worlds gathered together...!?

Trivia

 * All four (later five) series possess a theme involving the protagonist being transferred to another world.
 * Staff who created Pure Pure Pleiades (ぷれぷれぷれあです), Re: PETIT ~Starting Life in Another World from PETIT~ (Re:プチから始める異世界生活), Youjo Shenki (ようじょしぇんき) also worked together on this joint animation series.
 * In lieu of a chapter from the Overlord Manga, the series was promoted in the Monthly Comp Ace November 2018 Issue.
 * The MMORPG, MapleStory happens to host several Isekai Quartet events that starts on March 11th and ends on April 7th.