It’s scheduled for an August 31st, 2018 debut in separate formats where it’ll be in Japanese and German with German subtitles.
The light novel series began in 2012 and has ten volumes out so far in Japan.
With two seasons behind it and eleven light novels, we learned last year about the new OVA series under the header Departures for Testament of Sister New Devil.
The project came out in Japan on DVD and Blu-ray back on March 28th, 2018 and now German distributor Peppermint Anime is prepping their edition.
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The Testament of Sister New Devil - Departures - Clip #01 (dt.)