An army of darksiders seems pretty ridiculous after the Sith were destroyed. Where would all these darksiders come from? Too many lightsabers or force users in one place are lame anyway. The new Sith could be a secret project of Vader, frozen in Carbonite when Vader met Luke. Then, in Episode VII, he is thawed out. I doubt Palpy had a secret apprentice. He seemed happy enough with Vader in TESB. Remember, initially he wanted Luke killed, not turned.