Hi guys,
Just wanted to assert something else here, but I might be completely oblivious.
What about Sivirs Chalikar / crossblade being the Darkin? Now, I know Sivir has no quotes in-game to support this, and her backstory has nothing to do with the Darkin lore.
But the weapon, however, does not have a clear history. Sivir found it lying around in an old Shuriman tomb. Now, it does turn out to be some sort of key to open a certain chamber within the remnants of the ancient city. Cassiopeia knew this, she was seeking the ancient power allegedly held within and betrayed Sivir. This event brings us Xerath, Renekton and Azir. Also, after Azir saves Sivirs life, he begins restoring the ancient ruins to their former glory.
Now, we know Varus goes to Shurima looking for his sister. He doesn't recognize the lands to be Shurima probably because of Azirs handiwork. The story reads:
[Days earlier, a crook-backed creature with a withered arm and birdlike features had told him this was Shurima, but that couldn’t be true. The Shurima Varus remembered had been a desolate wasteland.
“Was I imprisoned for so long?” he wondered.]
And also:
[The new river had exposed the sunken ruins of some ancient structure comprised of pillars and statues of animal-headed gods. Yes, this was the source of the magic he had sensed. Old magic. The kind the flame-haired queen used to enslave his kind.]
Since he feels this magic within Shurima, it is acceptable to think that the queen that's spoken of is in fact Setaka, the ancient ascended queen of Shurima and also the original owner of the Chalikar. It's not unthinkable that she sealed one of the Darkin within her own weapon. In Sivirs lore it is stated that it was a particularly hard artifact to find, even for an experienced tomb raider like herself. It could mean that the crossblade was purposefully well-hidden, to keep the Darkin locked away.
We know Sivir is a powerful warrior with the Chalikar. It's possible it's power is greater because a demon is residing within. Since there is no "struggle" between Sivir and the demon, this theory gets into difficulties, although one explanation could be that this Darkin is not as vengeful as her brethren and does not seek to overpower whoever is wielding the weapon. Or maybe the demon has been weakened or is in some sort of dormant state.
What do you guys think?
??
.
. Since the rework on Shurima's lore we know that Brand was imprisoned in that ancient tomb before
and 
. Maybe the Fire Darkin is a female but she looks and talks like a male because she is possessing a male marauder's body, Kegan Rodhe. And that tomb could only be opened with a special key: The Chalikar, 
's weapon who once belonged to an ascended warrior queen from Shurima, Setaka. Why would she need a magic tomb with a lock for a simple fire beast?
