I don’t think you need the plague chest for anything. The breakthrough weapon is a stalker weapon from the fortune wheel from the first world boss and the 2nd to evolve is a lvl 20 blight weapon from the first dungeon and I believe that’s the same for every class. Plague isn’t Blight.
You still need a key for blight mind you, at least if you want a guaranteed one. You can just run the dungeon a bunch of times since you get boxes AND weapons drops from the boss, but RNG gonna RNG and you have to outbid people so might be slow.
They’re rare drops from viridian chests as mentionned, you can also buy them from the cash shop in the beta(not sure if it’ll stay like this live, viridian is only 20 NCoins on alpha) and lastly if you have premium active, you can buy a key for 20 viridian stones(the ones you get from doing various daillies), using the dragonsomething tab in your inventory.
I believe you can get some from the boxes that you get for doing dailies in that area (check the loot table to Buy Blade And Soul Gold). But they must be even rarer than the actual weapon so it’s better to farm those weapon crates.
Another possibility is to use the auction house. Try to sell the weapon you get and buy the one you need (if they are equally priced).
Not 100% on what the name is as I mostly played on the russian server, but it’s the town with a big river thingy and two dungeons one with a green blob that you get plague weapons from and the other has a big scorpion.
Just farm the blob until you get a plague weapon for your class and save the key for either the big dungeon or that quite rare blue you need to evolve it later as those cant be easily farmed.