There is an amazing story buried in these books.
A story of a Human-Forerunner war, and the humans are winning, right until the arrival and outbreak of The Flood. They fight the war on two fronts, and lose.
There's the political and literal battles between the Master Builder and the Didact - Halos and Shield Worlds - as Forerunner rulers debate the way to preserve the Mantle they have inherited.
Political intrigue, galactic warfare, enormous, dueling personalities and plans. A galaxy consuming organism, plans straddling preservation with total destruction, mysteries of the origin of life in the mix.
A trilogy about this saga could be truly incredible. But these books are not about this story. They are about a couple of fringe characters who periodically run into someone who knows about that story, and catches them up.
It's basically Rosencrantz and Guildenstern Are Dead, only without Hamlet ever having been written.
This describes both Cryptum and Primordium: A couple of characters go places because the voices in their head tell them to. They wander for a very long time, and then huge, mysterious things happen as ancient beings awaken or mingle for the first time in ages. Our characters are as confused as we are, until periodically someone catches them up to speed on what's going on.
Repeat.
Primordium is about 380 pages long, and I'm pretty sure nothing of interest happens on nearly 300 of them. Chakas wakes up, meets a couple of characters, and they start walking.
They walk for about a hundred pages, and then see a couple of interesting things in a ruined city. They walk for another eternity, and then run into a Monitor who catches them up on this amazing story they are missing because they are walking around a Halo ring. Then they run into the Didact, who further catches them up on what they've missed while they wandered around. Then something happens, and the book ends.
It was a really boring book, with incredibly uneven pacing. Important things are telegraphed in giant dumps of exposition, between huge swaths of nothing. There wasn't even any great world building to redeem the material this time around, as was the case with Cryptum.
I could go on - I really don't like the whole "everyone is doing what the voices in their head tell them" narrative conceit - but I'll call it good there.