I reckon we've seen other Darkin, we're just not aware they're Darkin.
Vladimir strikes me as a Darkin. Not Vladimir himself, but the power that transfers itself from one host to another as we've seen in his lore. We have no idea how long that parasitic power has been moving from host to host, or its origin.
From Aatrox, we've seen that his sword absorbs the life energies of others, while also sacrificing Aatrox's own life force to deal more damage. Vladimir's kit seems like a more primitive version of a Darkin sword.
I reckon it'd be a nice plot twist for a Darkin blade to be sentient, and that the power that possessed Vladimir was a Darkin 'blade' that was broken and freed, looking for a vessel to carry it.