Was it ever explained why they weren't seen in Halo 3 or why they were on the Ark? Aren't they promethean?
I don't have the book yet unfortunately.
Well, best I can put together so far is that the armigers aren't composed humans, but literal Forerunner AI constructs of some sort. The Prometheans are made up of composed lifeforms.
The book didn't go too deeply into where the armigers came from, or why they weren't present 2yrs prior when Mc and Co were on the Ark. It also doesn't explain where 000 Tragic Solitude was either... But maybe I missed those details. I'll have to revisit certain sections to see if I missed stuff, which is possible haha.
Could be information that they are saving for Halo 5, too.
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Edit: looked a bit deeper and I think a lot of it had to do with the destruction of the replacement Halo that heavily damaged the Ark itself. It's possible that 000TS wasn't as concerned as deeply as he should have been with the arrival of humans and Covenant on the Ark, until it was too late, and the destruction of it all caused him to enter a kind of delusional state. He takes control of the armiger forces, which were previously dormant, and uses them to defend the Ark in 2555.
The armigers are a new addition to the lore, so obviously they have to be shoehorned in to some degree. They have been around for a long time, and were used by the Forerunners during their war with the Flood. They are indeed completely robotic, but their creation process is a mystery,
save for whatever secrets Jul M'Dama has discovered, as he seems to have gained the ability to manipulate/use them*?
*a bit murky on this part.