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The X-nelian War is mentioned many times in the Lego Metroid series. The war began when Dorn invaded planet X-nel XI 5 years ago, wiping out major cities and caused the supposed X-nelian extinction. It is believed to have lasted as long as 5 years prior to the day the Galactic Federation received the distress signal sent by Dal-G, however, the remaining years of the war were supposedly dealt in exile by Dal-G himself, coming to conclusion that the X-nelian specie was killed off early in the war. As to Dal-G's statements about Dorn's status remains unsolved, it is believed that Dorn may have been defeated at one point during the war and connected himself to four distinct generators to recover.
The J-Yel Chronicles: A Lego Metroid Story is a chronicled mini-series said to be a prequel to Lego Metroid, that sets during these 5 X-nelian War years before and during Metroid Fusion leading into Lego Metroid.