A fresh phantom sea zone popped up this afternoon, so I played around in it. At first, there were lots of Wandering Skeletons up. At the same time, I could get no fallen warriors to appear from bone piles. So I slaughtered every wandering skeleton in the place, and then fallen warriors started appearing regularly from bone piles. It appeared very much as if the two were related, and killing wandering skellies advanced it to the next trigger. After that, I have no idea. I slaughtered a whole lot of fallen warriors. I tried dragging them around. My last thought was that to have a battle, you need a bunch of warriors, so I tried spawning as many as I could without killing them. No luck there either, although I couldn't get very many up at once before someone else ran them over.