From Halopedia, the Halo wiki
- 112 - "For fifty years, scattered through the galactic arm, humans probed our settlements and positions. Then they allied with the San'Shyuum, combined their knowledge, and created weapons against which my warriors had little defense." - The alliance between the human empire and the San'Shyuum was new at the start of their war with the Forerunners, and San'Shyuum technology provided weapons capable of breaching Forerunner armour.
- 131-132 - "The Didact requested scans of the planet's present biota along with lists of the flora and fauna that had survived the battles nine thousand years before. In the records of the survey conducted by the Lifeworkers, likely after the end of hostilities. I saw hundreds of species of larger animals ranging in size from a meter to a hundred meters-some clearly aquatic, others huge land carnivores or sedate praire-grazers. This list was compared with what the sensors could now locate.
One by one, the larger species dropped out.
"No animals larger than a meter," the ship's ancilla reported in a pecise, clipped voice.
Next came a range of historic species less than a meter in size-tree-hoppers, nurrowers, small carnivores, seed-eaters, flying creatures, arthropods, clonal sibling societies...the Pheru.
One by one, they dropped off the current list. None to be found.
Next came flora, including dense arboreal forests. Many of the original trees had acquired a kind of long-term intelligence, communicating with each other over centuries using insects, viruses, bacteria, and fungi as carriers of genetic and hormonal signals, analogous to neurons... That list also quickly emptied. There were remnants-dead forests and jungles covered with a false green carpet of primitive plants and symbiotic species
All that remained, apparently, were mosses, fungi, algae, and their combined forms.
"Nothing with a central nervous system or even a notochord," the ship's ancilla reported. "No fauna above a millimeter in scale."" - Forerunner technology is capable of reading life signs from orbit and determining their species just like in Star Trek. The ship's system compares life-signs with the record and removes species no longer in existance from the list. The extent of the Halo superweapon's power is seen to be capable of killing off all life forms with a nervous system rather than just sentient life. However, it is shown in this case that it cannot destroy all life, as primitive fauna without nervous systems are still alive and the non-sentient (in the M. Night Shyamalan sense) flora are still present.
- 248 - It took 3,000 years to design the Halo array and its weapons system. The Forerunner Council chose against the Master Builder's proposal for a field test.
- 248 - Bornstellar's father refers to the Halo array as "a horrible weapon that destroys all life-", but does not finish his sentence. The weapons system's 'tuning' has been changed to allow it to be more destructive if fired - the destruction of Precursor technology was not originally intended. Twelve Halos were originally commissioned by the Master Builder.
- 251 - Eleven installations are accounted for; one is missing (It may have been stolen by Mendicant Bias).
- 274 - "By radiating a powerful burst of cross-phased supermassive nutrinos, these installations were capable of destroying all life in an entire star system. Properly tuned and powered, they could do more than that-they could kill all neurologically complex life across whole swaths of the galaxy" - this quote says two things: the radius of each individual ring's weapons system can be arbitrality changed. The difference between "all life" and "all neurologically complex life" is very significant, and suggests that the array can target varying degrees of neurological complexity depending on its instructions.
- 274-275 - "The installations had been dispersed. The Halo tested at Charum Hakkor had been fired at very low power, acting as a test bed. That had been an unauthorized use. But then, a second Halo had been used to punish the San'Shyuum. With horror, I realized that what I had witnessed had been only the beginning-and that the San'Shyuum worlds, after our brief, traumatic visit, had been reduced to the awful condition of biological blandness we had seen on Faun Hakkor."
- 275 - The first confirmable sighting of the Flood by the Forerunners is around 300 years before the array's activation. The previous milleniae of actions were mere preparation.
- 282 - Eleven rings are brought to the Capital; one is still unaccounted for.
- 307 - "It seemed that after fourty-three years, the prodigal Halo had returned" - Mendicant Bias returns with the stolen ring.
- 313 - "The monitors are programmed to assume that all who attack an installation are enemies - whatever they look like, or whatever codes they possess." - 343 Guilty Spark's attack on John and Johnson is legitimised as having been forced to engage in defencive protocols. The red light does not indicate rampancy.
- 315 - "The Librarian's specimens-so many worlds are stored on the Halos, so many terrains and beings! What will happen to the fauna?" -
- 316-317 - "Mendicant Bias has turned against us. But I do not believe it has sufficient resources to control more than five installations at once. Other installations are following older instructions, priority protocols-they defend themselves, but are struggling to break free of the Contender's rule. They may reconnoiter outside our galaxy-at the Beginning Place. The Ark." - It would appear that at first reclaimers were not needed to control the array and that the rings' respective monitor was responsible for activating the array. Mendicant Bias is capable of controlling other ancillas, although they have enough personal control over themselves to attempt to fight back.
- 334: "On each of the installations, the Librarian's aides and monitors supervised the laying down of the Lifeshaper seeds, containing all the records necessary to create and restore unique ecological systems on the inner surface of each ring. I could see evidence of their work even from where I stood-mottled patches of early-stage jungles and forests, the tan of desert, sheets of ice...
Earlier, when I had voiced puzzlement at the contradiction of Halos supporting these living records, my nurse and guardian, a Lifeworker named Calyx, explained that the Librarian had equipped most of the Halos with living eco-systems, and stocked them with many species from many worlds-selecting from those multitudes that been gathered over the last few centuries, and now populated the Ark's great half-circle."
She had hoped to preserve many more species by using the Halos; the Master Builder, after aggreing to her plan, had decided it would be useful to test captured specimens of the Flood on the Halos before they were fired-to learn more about them.
Sacrificing those populations, of course." - this is the explanation of why each ring supports life despite contradicting their purpose.