Disco Mental Institute

[SparkyRailgun]

Oh god dammit, more crazy unprecedented backstory.

In what expanded-universe-comic-book-or-something-that-I-can't-be-bothered-to-keep-up-with-anymore was this tidibt revealed?

My understanding was that it was some extra-galactic parasite that suddenly appeared to the Forerunners and killed 'em off, but the Forerunners locked some up on various installations before they blew everything up, and then the Covenant came along, derped it up on Installation 04, released the Flood there, and then did so again and again on various other Forerunner installations as necessary to the game-plot.

At what point did "Ancient Humans" have access to Flood-cells, anyway?

It is extragalactic, to an extent.
Humanity first found these cylinders hundreds of thousands of years ago (they were an intergalactic empire allied with the Prophets, go figure). After Humanity devised a method of destroying the Flood (turning it against itself, in essence), a surviving portion fled the galaxy. Human disdain for the Forerunner's dominance over the galaxy turned to war, with the Forerunners winning and literally bombing Humanity back to the stone age.
This Flood took this as a great opportunity to return, and thus began the fall of the Forerunners, and the first activation of the Halo array.

I may be slightly incorrect on some points (working from memory), but that's the basic jist.

EDIT: And to answer your first question; Books.

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