Task: Question Generation
Input (English): Context : Ragnarok DS, known in Japan as Ragnarok Online DS (ラグナロクオンラインDS), is a Nintendo DS video game based on the MMORPG Ragnarok Online and was released in Japan on December 18, 2008. 
Subject : Ragnarok DS 
Relation : series
Output (English): What series is Ragnarok DS part of?