Honestly with all the lore we have compiled over the 20 years the game has had, the reading list for new comers is daunting but ill refer you to my friends over at “Halopedia” (won’t let me link the cite here in the forums) which has a bunch of great resources and information regarding lore based questions. Other than the basic summaries you get from playing the games, and Halo Legends, the tldr from the books (skipping all the forerunner background and the first seven halo rings, plus the origins of the flood and the precursors and security measures mantle, and all the crazy lore around that stuff) There are currently Five Rings, a Destroyed Halo Ring, and a Damaged Zeta Halo Ring as well as a mini back up halo ring that was released form the ark via Jerome and Isabell from Halo Wars 2 that are in the universe. The main Original Six of the new Halo Array (zeta Halo I believe is just as capable but the last remaining of the original array and I believe is less powerful) can detonate and essentially erase all intelligent/sentient life cutting of the floods foods source, which is basically intelligence. Its both the easiest and most complex question to answer xD let me know if you have other questions I’d be happy to link you to a Halo wiki that will explain!
Here are some things you should search on Halopedia:
Halo Array: [Halo Array - Halopedia)
Forerunner: [Forerunner - Species - Halopedia)
Flood: [Flood - Species - Halopedia)
The short answer is that the Halos can be tuned narrowly enough to kill just a specific species in a single star system, or as broadly enough to kill everything with any semblance of neurological structures in a 25,000 light-year radius per Halo.
The information revealing this is dotted throughout the Forerunner Saga. Bornstellar and the Didact visited a world where a Halo was test fired, and it killed everything, including sentient plants and everything else except moss.
The Halos do kill the Flood. This is seen in Halo Silentium and in the “Origins” videos from the Halo Legends series.
From what lore nerd youtubers said off the top of my memory they fire off an energy wave that fries nervous systems and the flood are incapable of doing anything but “still live”. After the arrays are fired sentinels would go about and sweep clean the infected planets/ships/etc to reset the galaxy? Animations depict it as directly destroying the flood but said youtubers said that depiction is incorrect but idk.
As far as I am concerned I think it would be silly to somehow develop a weapon that kills everything but the flood why would you not include them of all things when developing a galaxy wiper.
@Greviator This is the closest explanation here, but to be more specific on the “how”; a Halo essentially fires a wave of exotic particle radiation that travels through sub-space and, as mentioned by others, destroys anything with a neural network - animals, certain plant life, the Flood…anything with a nervous system is wiped out.
Spores can survive the firing I believe, however given that firing the array would wipe out anything with sufficient biomass by proxy (certainly nothing with a neural pathway) then those spores will either lay inert or be unable to do what they normally would past a certain point.
Given the way the array works, it also destroyed Precusor architecture as it was all based on Neural Physics. The Domain itself was believed lost due to this…however, as revealed in H5…it survived.
To expand on that, the Domain is the memory of a Precursor construct named Abaddon (and incorrectly remembered in Forerunner history as the Organon), and when the Halo Array fired, destroying the neural-physical structures that supported the Domain like scaffolding, it effectively wiped Abaddon’s hard drive, and stripped away most of its processing power. It was rebooted, enabling it to rebuild itself after the Array fired (though the information previously stored in it was gone), when Bornstellar led a team to Maethrillian to do so.
Actually, that’s not necessarily true and is a point of contention.
The only point of reference we have of Abaddon and the Domain being one and the same, is the Gravemind telling the Librarian this in the final moments of the war before the Array was fired. Given the nature of the Flood, this may well have been a lie/ twisting of the truth.
Abaddon was in effect an AI created as the Overseer/ Keeper of the Domain by the Precursors. It makes more sense that it existed alongside the Domain, possibly within it in a way, but I would contest that it WAS the Domain. This is backed up by the final events of the mission you highlighted, where the deadbolt was used to restrain Abaddon, with the user sacrificing themself to be used as a “template” to restart/ heal the Domain.
It’s possible that she now exists as Abaddon did and given that she’s female…maybe she is the Harbinger??