The fantastic Horizon Zero Dawn came to PC this week, letting a new audience experience a game that had previously been a PS4 exclusive. With the sequel Horizon Forbidden West announced earlier this year by Sony for the PS5, now is a great time to catch-up on the series and see how the ending teases out the game to come.

Horizon Zero Dawn released in 2017 for PlayStation 4 and was a commercial and critical success, becoming the console's best performing game up to that point. Set in the 31st century, it followed Aloy, a young woman trying to discover why the robotic beasts that roam the world have recently become corrupted and hostile. In doing so, she uncovers the truth about a world-ending apocalypse and the program created to bring life back to earth.

Related: Horizon Zero Dawn PC Review: Still Great, Less Exclusive

Horizon Zero Dawn ended with Aloy defeating HADES, a corrupted AI who believed its purpose was to wipe out all life on Earth. To do so, HADES planned on reawakening a group of robots, known as the Faro Plague, who were responsible for ending the world originally. Aloy was able to shut down HADES and prevent it from enacting its protocol. However, a post-credits scene showed that HADES was dormant, not dead. In a flash of red light, the AI exploded out of a dead machine and zipped through the air, summoned and trapped by a device wielded by a man named Sylens.

Who is Sylens and What Does He Want?

Sylens looks up at something offscreen in Horizon Zero Dawn.

Sylens has a complicated history. Although in Horizon Zero Dawn he helped Aloy multiple times in her quest to defeat HADES, Sylens was always protecting his own interests. It was revealed that he had long ago discovered HADES and for years fed it information about the world in exchange for knowledge about technology and physics that had been lost to time. It was only after HADES betrayed Sylens by trying to kill him that Sylens started working against the AI. Even so, it's clear from his capture of HADES that Sylens still wants to use the AI for his own gain. During that post-credits cutscene, Sylens muses that he never did find out who corrupted and awakened HADES. He then turns and reveals a MASSIVE (currently inactive) robotic titan.

This post-credits cutscene teased out that there were still secrets in the world of Horizon Zero Dawn. As Sylens points out, someone corrupted the HADES AI, someone who is never discovered by Aloy. The trailer for the sequel game, Horizon Forbidden West, shows that a new threat has arisen — but it's a threat that comes with the familiar red-tint of corruption. Is this HADES once more, or a new AI? After all, the DLC expansion The Frozen Wilds had Aloy fighting against an agent of the AI HEPHAESTUS, revealing that there was at least one other AI out there dedicating its resources to destroying humanity.

Whatever the threat, Aloy appears to be heading to San Francisco in pursuit of answers, and she'll cross paths with Sylens along the way. Sylens can be spotted briefly in the Horizons Forbidden West trailer, overseeing a group of humans who appear to be cleansing a corrupted robot. What knowledge Sylens gained from HADES and how he intends to use it (and that giant robot) will be a core mystery in the Horizon Zero Dawn sequel. Only time will tell if Sylens will ally with Aloy once more, or if he will join the mysterious entity that almost ended the world for a second time.

Next: Horizon Zero Dawn's PC Features Are Exactly What Players Want

Horizon Zero Dawn will release for PlayStation 5 in 2021.