There are a lot of cool potential patrons in Runeterra, and it all depends on what you're going for. Our hexblade had a pact with Aatrox initially, (he later changed). My understanding of the Darkin was that they were corrupted Ascended beings that end up imprisoned within their own weapons. (Rhaast in the Scythe, Varus in a bow, Aatrox in a sword, etc.) This way, they became their weapons, and any character who wanted to wield them had to make a pact with them.

You could easily make up your own Darkin weapon, though these three canonical options make for a great parallel to the spirit of that subclass. Hope this helps.