Warning: major spoilers ahead... obviously.
Metroid: Samus Returns released earlier this week, and players who have already cracked the 3DS game know that it features a post-credits scene that points towards the future of the series. However, the scene that plays out for those who have 100% completed the game has now been revealed, and things look set to get a lot more challenging for Samus Aran (and vastly more exciting for the series' fans).
In the post-credits scene, an alien can be seen being infected by an X Parasite, spelling a great deal of trouble for Samus and the rest of the galaxy. The X Parasites have previously only appeared in the Game Boy Advance game Metroid Fusion and the Metroid manga, but suffice to say they are a deadly enemy in Metroid lore, with them having previously overrun SR388, the home planet of the Metroids.
In Metroid lore, the Chozo existed roughly 1,500 years prior to the events of the game series. They were responsible for creating the Metroid in order to rid the galaxy of the X Parasites, unwittingly spawning a new threat in the process. Up until this point the fate of the Chozo has remained a mystery, one which may have finally been uncovered by the "true ending" of Metroid: Samus Returns.
If players achieve a 100% completion rating in the 3DS game, they're treated to a series of images that appear to lay out where the Metroid series is headed, along with offering some insight into the history of the once mighty Chozo. In a series of screenshots posted by NeoGAF user Cerium, the first Chozo voyage to SR388 is revealed, showing three Chozo leaders stepping foot on the planet.
Next the Chozo are depicted extracting X Parasites from their hosts, seemingly in an effort to rid the planet of the destructive creatures.
They're also shown performing scans of life forms on the planet, presumably as a method of detecting X Parasites.
More X Parasites using aliens as hosts. The alien in the foreground is identical to the one featured in the post-credits scene.
Now here's where it gets super interesting for Metroid fans: in the below image, the Chozo are shown designing a Metroid using their advanced technology.
More and more Metroid are created to attack the X Parasites.
With the X Parasites gone, the Chozo are free to live in peace...
...until the Metroid start mutating and overrunning their creators.
The Chozo can no longer control the Metroid threat as their civilization comes under threat.
Finally, a discussion is held between two Chozo leaders...
...which leads to one Chozo leader seemingly killing the other, standing triumphantly over its body while overlooking its army.
The demise of the Chozo has never been detailed in the Metroid series, though it was always suspected that they had been wiped out by the Metroid after creating them. However, the sequence of events depicted in Metroid: Samus Returns appears to show that the Chozo had split into two factions after the Metroid began overrunning SR388, with the leader of one faction killing the other as part of a civil war.
Now the question remains: where is this evil Chozo faction? With a live Chozo having thus far never appeared in a Metroid game, this ending suggests that Samus is set to come up against the ancient race in the future, which will be a huge deal for long-time fans of the series. Though Metroid: Samus Returns follows many of the same story beats as its predecessors, this is a considerable shake-up for the franchise, and will no doubt leave many eager to find out where it'll go from here.