Maybe overall it could be Gravemind, an ancient entity that has it's own (twisted) morality & manipulative nature.
It being that old brings a bit of mystery about it's origin & motivations, is it just revenge story about the destruction of species it once represented, is it truly a seek of unity in mind & body of all sentient beings as part of some flood utopia of galaxy wide peace, is it simply just lies of an ancient parasite creating moral doubt in the mind of sentient creatures utilizing the name of ancient creators... so many options.
"Do I take life or give it? Who is victim, and who is foe?"
And the old lore about humans being forerunners & the story of seeking the place as most powerful beings becoming the downfall of the galaxy wide civilization along with everything else after they destroyed precursors whose attempt at second chance becoming corrupted which resulted in unintended revenge & another meeting between the 2 sides after the destruction of sentient life eons later, usurpers having started another rise towards the power & the creators being reduced to this entity of this collective hate inside an abomination of a physical form.
"Child of my enemy, why have you come? I offer no forgiveness, a father's sins, passed to his son."
And maybe even this Dracula type collectiveness when in need of manipulation but revealing the monster inside when found having the upper hand.
"Time... has taught me patience! But basking in new freedom, I will know all that I possess!"
Several others have their moments too but such beings as Sesa & Tartarus are these pawns in others game, even though they are great warriors of their time they are either like infants next to ancient beings or being guided/manipulated by others.
Truth is a leader & manipulator of collective union of space faring species but it is just one of many such leaders, a moment in face of eternity, like many others before in campaign to form & drive the covenant. A bright flame perhaps but brief & almost hidden among the others.
343 guilty spark, an ancient being & powerful even, utilizing eons of knowledge & control over machinery but simply a pawn left without a master gone mad & manipulative, a tool though one with presence much like Gravemind in a way they know things from far away, gravemind through biological hivemind & spark through tier 1 technology.
So I would say it's through most aspects the being that potentially possesses knowledge from past ages spanning from the time of the precursors to the events of the ark when modern humans first found it. in addition of possessing the power to more than match the most (if potentially not all) advanced living beings in known universe.
"Resignation is my virtue; like water I ebb, and flow. Defeat is simply the addition of time to a sentence I never deserved... but you imposed."