My favorite theory is that fallen were just like guardians, but know they need Servitors(sorta like ghosts) to keep them alive.
My personal headcannon is that the eliksni didn't look the same at all before the traveler came. They were made into immortal husks(kinda like guardians again)
But since the whirlwind, they need a new source. There's some evidence for this but nothing concrete
I do need to read the book of sorrows lol
English
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Ether is a light substitute. The traveller/Great machine showered their worlds in Ether, and now they subside on an artificial light source after the whirlwind/collapse.
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https://www.bungie.net/en/Forum/Post/153754344/0/0