It always feels like Bungie made a story from 2001 to 2004 that was too perfect, and that from then on after, they’ve been trying to retcon how to proceed.
I don’t necessarily know if the Forerunners were wrong. What I took from the campaign is that the Forerunners wanted to lock away the Xalanyn and bury them so deep (both figuratively and literally) such that no one would ever find them. They chose not to catalog the Xalanyn before firing the ring. They chose to completely wipe the galaxy clean of the Xalanyn and then pretend they never existed… The Xalanyn inexplicably reappeared and the Forerunners had to scramble.
They sound pretty bad, however comparing them to the Flood seems a little premature. It’s almost like comparing a rookie quarterback to a hall of famer. That being said, there’s much more to this story that we are unfamiliar with, at least for now.
I dunno much but I wonder if the Endless would send their most valuable soldier anywhere without max upgraded shields and equipment by default…
Here is another question that I thought about. Would the precursors refer to themselves as such? We know they came before the fore runners and were also killed off by the forerunners before their corpses reanimated into the flood. My guess is the forerunners called them such so no one would go looking for them. Even the audio logs by the monitor hint at this. The other reason this is brought up is 11 out of the 12 original rings were destroyed before Faber made 6 replacements at the lesser ark. During Halo 5 Cortana quotes the Ur Didacts epilogue speech from Halo 4. However, she says that those who refuse her aid and cling to their old way their will basically will feel her wrath and when you are gone “we will take what is left and rebuild it in our own image…” The reason this is important is due to the Ur didact also being corrupted by the gravemind. So here we have Cortana, The Harbinger, Medicant Bias who was in contact with the gravemind and the Ur Didact all having some connection to the gravemind. Thus, the endless are precursors who wouldn’t refer to themselves as such. So given what we know my guess is that the precursors made the original 12 rings before the flood was ever a concept. they were hunted to extinction and imprisoned. after they were killed off just like to Domain or the Mantle, Forerunners claim to be its master when in truth they were thieves. Based of all the lore from the canonical halo wiki page that my assumption of the new threat. For additional context Lucas Browning, the Marine who was used to release the Harbinger mentally went insane in the audio logs. this is similar to the first humans who asked the primordial about a cure for the flood who went insane. However, Lucas says something interesting in one of the audio files. “Closed space… Off limits… Still there… They were still there… the only one”. My impression of this is the edge of space where the corpses of precursors turned into the flood via dust. him saying that they are still on the edge of our galaxy. The harbinger is aware of the human-forerunner war. She also talks about how the humanity was the forerunners rivals. How their kind was not spared because they couldn’t be controlled. During that time their were only one other race that was on par with humanity and the forerunners. that was the San shiyum but they were defeated so this once again brings me to believe their referring to the primordial.
I think just the fact that you had a race of beings that were unaffected by the Halo Rings and, thus, would’ve had 100,000 years of uninterrupted growth/evolution while the entire galaxy had to start over could’ve been a rather serious problem right then and there so there is some justification for a measure of containment.
As to what made the Xalanyn so dangerous that the Forerunners wanted to literally lock the doors on them and then throw away the key remains a mystery but I could see some potential advantages.
For starters, while the Flood had nearly been able to consume the entire galaxy, a big part of that was due to being able to incorporate not only Forerunner but Precursor knowledge and technology, the most infamous example being how the Flood had been able to manipulate the Star Roads - something that the Forerunners hadn’t been able to do - and weaponize them to such devastating effects. In this current era, the technology and beings that they can consume and learn from are the likes of humans and the Banished/Covenant races and, when caught in the early development stages as we’ve seen in Halo Wars, can be readily defeated.
The Xalalyn, being an advanced race in those bygone era of the Forerunners, can potentially single-handedly upset the current power structure of the galaxy. And depending on whether or not that they had been edible to the Flood, the fact that they survived the Halo firings can potentially lead to two things: one, that they were as immune to the Flood as they were to the Halo Array. Or, two: they had been part of the Flood, had been freed of the hive mind when the Halos fired, and now they possess the same frightening knowledge to access and use all manner of Forerunner and Precursor tech.
Just a couple ideas.
I think what we need to take into account about the endless is what does ascension mean to them
God reading that makes me realize just how over convoluted this story has become. I’m all for growth and change, but these games used to be about saving the world to rock and roll and we’ve really kind of lost the spirit of that.
Too much lore can become suffocating imo. I just want good characters, a story I can get behind. I want that old “Hoorah” feeling this franchise used to give.
Military sci-fi always seems to fall into these tropes. I don’t know anymore. How many times are we gonna have to hear “We can’t let them activate the ring”? It’s just old. Chief’s story was tied with a bow in halo 3. They didn’t kill him. They just put him back to sleep to fight again another day. Which was awesome. He never needed to come back.
Chief isn’t a superhero, he’s an eternal soldier. He’s the idealization of a warrior compacted into one man.
Maybe it’s just time to call it quits on all this. I don’t know. Makes me feel kind of depressed.
And he didn’t even mention the fact that Infinite uses the iconic Halo music to paint a weird love story… They lean on the OG music far too much in general. It’s a cheap trick they also used in the new Star Wars sequels, just play the music people love and make them feel stronger emotions like the movie is giving them more than it really is…
Sad, I made a Post asking for the same thing with zero replies 
Anyways, I think there is a lot that doesn’t make much sense right now and which can’t be answered with any lore we have.
For example: I think the Xalanyn and Endless can’t really be Precursors. The Precursors created life and therefore were no true threat to it. Also, they were actively looking for a successor, so they weren’t keen on keeping power. Lastly: They deemed Humanity as worthy to hold the Mantle. Why should they create a third species to take over then?
The only thing that would somewhat fit in this would be some sort of “Precursor Splinter Fraction”, which saw things differently.
But even IF: What makes this so terrible, that time need to forget about them?
I mean…the Foreunners declared war on the Precursors before and wiped out the whole galaxy because of the Flood - they made no secret out of this. What could be worse? One war or one species more or less…
The fight against the Xalanyn didn’t seem too hard for the Foreunners. It was not a century-long battle. Seems like they were contained quite quickly and with ease. And Master Chief also defeated the Harbinger with regular weapons. Soooo…they are not immortal 
I also think that the Xalanyn and the Endless are not one and the same. Harbinger talks about the Endless in third person (Not “we”, but “they”). To me, it feels like the Xalanyn were some kind of “keepers” of the Endless, while the Endless were the actual threat from a million years ago. Kinda like the Prophets more or less released the Flood.
So in the end, I doubt we can derive all too much with the current knowledge we have. We can rule certain thing out and say what they are NOT, but in the end, I guess they are something completely new which was never mentioned in any book or game before.
Edit:
Oh and one very important thing: The game highlights that the Xalanyn were mistreated. They were judged without trial and without doing anything wrong. The game also highlights the odd morality of the Foreunners because of this. “If” they were objectively a threat, the Foreunners did everything right.
Quite interesting is also the line “They believe we are here to help” in the legendary ending. This really makes me wonder.
She didn’t. Atriox did, for reasons yet unknown. I assume it’s to tear down everything that the Forerunners built, and essentially “reset” the Galactic order.
There’s a cutscene where we see her demanding Despondent Pyre give her what she wants. The Monitor says that Punished who she says can never be freed and that the consequences of her request are dire. Cortana asks her to reconsider before dismissing the Monitor.
I had forgotten about that context. That is curious, and a little frustrating as we likely won’t get answers as to what she wanted the Xalanyn for.
You don’t think we’ll see more flashback cutscenes in the DLC?
It would be odd if she didn’t speak with Offensive Bias if he’s on the ring and if she knew more about this threat why wouldn’t she also have left some clues regarding that?
Plus it would be criminal if the Flood do show up and we don’t get a rendition of the Gravemind corrupting her.
I would certainly hope so. To be honest, I’m a little disgruntled that the storyline for Halo 5 was dropped so quickly; one of few criticisms that I have of 343i is that they bend far too easily to the angry fans that want changes just because they don’t like it.
I just find it funny that people complain about the space magic when Bungie went off and did the most OTT space magic setting you could go for. Like I watched a video describing what the Vex were and my head just melted.
Yeah it does feel odd with them course correcting so heavily. It is a story in progress and there’s some odd story beats going on; so I am partly reserving judgement. I mean if the Chief gets in a time machine, goes to Genesis and actually does speech prep this time. 