Not much lore on this champ yet, but based on their choice of weapon and armor, it seems to me they COULD be a ghost that originated from Mt. Targon.
Could the black spear be a manifestation of their hatred, or might it have been a Relic Weapon like Pantheon's Shield?
That was my first thought. She looks very much like 
and the spear protruding from her stomach may lend itself to the idea that she was betrayed by her own forces in battle.
All I know is, this girl has serious amnesia and multiple personality disorder. Don't believe me, here's her VO https://www.youtube.com/watch?v=7TTS8gMhvRw
I thought this too because of the Greek name and apparel, but reading her quotes when buying 
made me doubt :
"Once, he was a noble king, until grief overcame his reason." "In the end, it was not his blade that stole our lives." "As once it symbolized hope, so now shall it incite despair!"
So, since I do think she has to do with Rakkor, I'm inclined to bet on some retcon tying things together? Or maybe the Rakkor thing was her start point and then in development they saw her as more befitting of the Shadow Isles.
Maybe Hecarim 
betrayed Kalista? He was shrouded in mist, but we don't know what his story of betrayal is.
Some theorize that he betrayed Mordekaiser, but the altar quote is "Mordekaiser is not to be trusted, Hecarim!", which suggest it would be the other way around.
Centaurs are similarly inspired by Greek mythology as Kalista's amazonian themes. (Maybe they are both related to a Targon from ages past? People vs. (now extinct-) centaurs?)
Hecarim uses a polearm, which is similar to a spear. He is similarly suggested to not remember his former life much ("Remember who you once were, Hecarim!"), but maybe they knew each other when they were alive? Hecarim is pretty old though, so Kalista would have to be too.
If she wants revenge on Hecarim that might be her ulterior motivation for her getting so busy with vengeance. Their shared Shadow Isles affiliation or Hecarim's relation to the Black Mist (as he leapt out of it in the cinematic) might then be an obstacle to her at the moment.
P.S. It's interesting that Kalista seem to be familiar with the story of the Ruined King, almost as if she knew of him in life. I wonder what the implications of that are?
Alright, I have been examining rakkor style of armor and weaponry by cross examine pictures and trying to see if the same style is incorporated. I can say for sure that she is not from Rakkor. Her armor is way too complex in its design. They use single sheets of metal to create their armor and their plates do not often overlap. They stick to only a few colors to where as her armor uses many. She does not use strip plating like the Romans or single plating like the Greeks. They also made their weapons to be simple but, effective nothing overtly fancy like gut hooks. I just don't see enough Greco-Roman.
"Ferocious Warrior" Killed in combat, wields a spear, seems to have been impaled by spears as well.
I think it's a strong theory. Good eye.
She does look like it,but I think it might also be just coincidence. However looking at the black mist in the video,she may have been a rakkor who was claimed by the shadow isles,which would be indeed awesome(because so far we've only had champs from the shadow isles who willingly joined them or lived there before)
If this is true, then it could be coincidental that Riot read my idea about a Targon champion that was supposed to be Pantheon's rival and then claimed by Shadow Isles. But it looks like they just missed out my latter idea where Diana rescued the champion, causing the latter to pledge loyalty to Diana and her cause for the Moon.
Diana. Forever alone.
EDIT: Never mind. The VO seems to have her speak with Karthus and Lissandra. Probably the ones covered in Black Mist during the Harrowing.
She's getting a bloodmoon skin; I thought that was specifically for Ionians
Also I find it interesting how Varus and Kalista have a relatable title. Arrow of Retribution and Spear of Vengeance. Are we going to hear about the Sword of Demacles next?
I just read the wikia's Skins and Trivia and it seems that Kallista's name is derived from a Greek myth figure, Callisto.
And isn't it Targon's thing to base designs from Greek/Roman myths?
Mount Targon is the Parrot's territory. She doesn't have nearly enough armor to qualify.
I don't see the connection between Pantheon. I see like Ionian almost like an ancient Xin zhao. Look at the style of the spear head and her helm. Look at the ponytail hairstyle it's seen multiple times in Ionias champs, its loose flowing and it's also very dark another Ionian feature. I am almost positive she is from Ionia.
So she is related to the ruined king then. Cool. I'm actually pretty happy they are delving further into this.
she has a lot of hate for varus and betrayal in her VO
did she give varus the power?
Now that we've got her voice over I doubt that she is from the rakkor,as she makes no mention about them and seems to remember the ruined king before he turned the shadow isles into a land of the dead.
If she is, it supplements my theory.
I would guess yes based solely on her appearance.
I get the impression from her voice quotes that she says "we" referring to her and her soul-bound. And she just gets used to saying "we" a lot, and needs to correct herself at least once. Not like that's a big deal.
Kinda like when a champ says "let's go blow 'em up!" The word "let's" is short for "let us." It doesn't imply that they're suffering from multiple personality disorder, it just means that they're on a team, like all champions are, all the time. unless there are a LOT of people who disconnected that is.
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