The 4th or the 5th Darkin will not be a weapon, but scales. Also, the champ will be a mage or supp

Not animal scales, but the tool you use to weight stuff, very likely a medival variant of it. Why would I say such nonsense? Well, the current Darkin all currently follow the trope of the Four Horsemen of the Apocalypse. They ain't even subtle about it: when the five Darkin ride, the world will burn. At least they will at least try to do that.

Now the four horsemen come with certain iconic weaponry: the first rider is Pestilence, who wields a bow. The second one is War, who wields a sword. The third is Hunger, who has the scales. And the fourth finally is Death, who wields the typical iconic scythe. Don't three of those sound awkwardly familiar already?

Here's also a short refernece:

https://de.wikipedia.org/wiki/Apokalypse_(D%C3%BCrer)#/media/File:Durer,_apocalisse,_04_i_quattro_cavalieri_dell%27apocalisse.jpg

Now the scales were here used as a symbol for the merchant, who would make his food so expensive none could buy it and therefore strave to death. At least that's how he fits into the group thematically with the whole "things that killed people in the middle ages" kinda stuff.

But because you can't fight with some scales, this champion will most likely be a mage, either as a support or as a midlaner.

At first I thought it might be coincidence, but with 3 of the 4 iconic weapons being shown in the same demonic faction, that's not just a coincidence anymore and it's obvious what overarching theme Riot is rolling with this here,