Darkin lore is still pretty vague, but there are suggestions that the Darkin were not always "sentient weapons", but rather free beings that became imprisoned in the weapons. We don't really know what a "free" Darkin is like, but they are surely immensely dangerous and powerful, and they must have existed a long time ago for them to be so completely lost to history.
This theory is supported by a few lines in Kayn's voiceover.
> Rhaast: "That weapon was so confining!"
> Rhaast: "The weak run. Stay and fight"
> Kayn: "Did such foolishness lead to your imprisonment?"
If this is the case (and for the sake of this thread I am assuming it is) then it begs the question of how the Darkin became trapped in these weapons. I theorise that it is Mordekaiser who is responsible.
Domination and enslavement is his _modus operandi_. Usually this takes the from of enslaving human souls, but it would not be out of character for him to enslave some ancient death-machines by trapping them in weapons.
Mord is one of the oldest (if not the oldest) Runeterran we know, so it is plausible that he existed at the time of the free Darkin. He is also an immensely powerful mage. Other Shadow Islanders get their powers indirectly from the Black Mist, but Mordekaiser existed in his current undead form even before the Ruination. So he may well be magically powerful enough to trap the Darkin, as well as knowledgeable enough to know how.
While hardly evidence on its own, Rhaast the Darkin Scythe was (re)discovered in Noxus, the old home of Mordekaiser. Perhaps Swain unearthed one of Mord's old armouries?
Bonus theory: Nightfall could be a "dead" Darkin being used by Mordekaiser, just like how Kayn kills Rhaast to become the Shadow Assassin.
This is admittedly a bit of a random theory, and we are unlikely to find out more about the Darkin any time soon. But I think it would be an interesting addition to Mordekaiser's lore.