Comments on the guide for Dark Sectors in Warframe by JTJDG666

What misfit119 said is spot on. I had a slightly different experience though. When I tried defending on a dark sector, it wouldn't let me. It kept saying that it couldn't find other players, so I only attacked.

I won five times I attacked, and the achievement didn't pop, but my progress bar went up after every win. After the dark sector conflict was over, the clan that was defending was the victor, but my progress bar stayed up.

The next time that clan had a conflict on the same planet, I fought again for the attacking clan again, it was a different clan that was contesting the sector than before. I fought and won only one battle for this clan and the achievement popped after the win.