OBVIOUS SPOILER WARNING HERE: As you might expect explaining the Horizon Zero Dawn ending means spoilers. So if you've not finished the game yet, come back later.
The Horizon Zero Dawn ending finally sees Aloy defeat Hades, the life eradicating AI that's been trying to wipe out all life. Although she doesn't. After apparently killing Hades, a subsequent cutscene shows some sort of red energy leave the broken AI core and fly away to meet Sylens, who captures it in a lamp like object and greets it like an old friend. Hades isn't dead then, and Sylens is suddenly a lot less ambivalent as a mysterious mentor to Aloy.
Obviously this was a set up to a sequel, which at the time didn't exist. Now we have Horizon Forbidden West. While the announcement trailer does little more than tease, it does seem to continue some themes from Zero Dawn's final moments.
For starters Sylens is back, apparently overseeing the corruption of machines with a group other people - the red eyes and mechanical tendrils from the process are unmistakable. It suggests that Hades, originally behind the process, is back in play somehow after Horizon: Zero Dawn, but to what degree isn't clear. Was that red glowing energy all of Hades or only a part? The ending of the game muddies his motivations somewhat - while he was pivotal in bringing Hades down, he says right at the end that he has much more to 'discuss' with the AI, as well as questioning after its 'masters'.
Technically Hades' masters would have been the original human race (us) before their autonomous machines glitched and started to consume all the biomatter on the planet (also us, along with every plant and animal on the planet). Hades was originally one of nine subsystems within Gaia, who was designed as part of the Zero Dawn project to restore life post-apocalypse. While most subsystems were focused on detoxifying the planet and repopulating flora and fauna (as well as one solely designed to hack into and deactivate the rogue machines), Hades' job was to reverse everything and scour the planet clear for a do-over if things weren't working.
Horizon Zero Dawn technically starts when a signal of unknown origin split Gaia's subsystems into separate, self contained entities. Hephaestus, the system responsible for making machines, started making them again, and Hades followed its core, only, function - wipe life from the planet. That initial event is what triggers Aloy's birth as a clone of one of the original scientists involved in Zero Dawn and, as a result, genetically compatible with a range of security protocols involved in stopping another robo-apocalypse.
The ending, and hints from the sequel teaser, pose a lot of questions. For example, is the weed like red blight we see killing plants and animals anything to do with Hades' programming to wipe out all life? Or is it anything to do with any of the other AIs loose in the world? Sylens' activities in the first game suggests he's pro on the idea of Hades not destroying the world, so is he after any of the other AIs, or the knowledge they might have? The original plan of the Zero Dawn project that spawned Gaia, was to reinstate man at the technical level it was destroyed at, but a decision was made to erase all that data. Without it mankind wouldn't be able to repeat the mistakes of the past but would be sentenced to a primitive life.
The title of the sequel, Forbidden West, isn't touched on in the ending of the first game but it is mentioned. It's an mysterious area that few people return from. Most of the mentions in-game suggest a dangerous location full of new machines and tribes but, with so few people ever returning, little is really known. However, some descriptions of the area could suggest it was attacked with nuclear weapons. That might mean it was important during Operation: Enduring Victory. This was a largely pointless counter attack against the robots that served a duel function in buying time to complete the Zero Dawn project to repopulate the Earth post extinction, while giving the general population a fruitless hope it would be avoided.
If the Forbidden West was important enough to be nuked during Operation: Enduring Victory, and Sylens and Hades are there now, it suggests it could all be to do with the old ones, Gaia, and more Zero Dawn stuff. Hopefully a new trailer will fill in some gaps and the conspiracies can begin.