I also thought it was Nightmare mode, but I got the DS to spawn on some level while playing HNTR, and the Berserker armor spawned in the Lava Pits.
It's possible for DS and BA to spawn randomly regardless of difficulty, but you need to play Nightmare! to get the guaranteed ones to spawn. (As far as I know)
I know that there is a specific way to get the DS. However, here's what screws me over: don't you have to get the Berserker Armor FIRST and then you can pick up the DS? So why can 2Dev pick up the DS on UC and then get the Berserker Armor on the Mortuary, which comes afterward?
From what I've observed, I don't think it matters what order you pick them up in, but you need to wield DS before you wear BA.
Also, attempting to pick something up counts as finding the item. If you don't attempt to pick up the DS, it can spawn again later that run.
That was certainly true in previous versions, but apparently, they've changed it in this version so you only get credit for finding it if you successfully pick it up. :| Now, even if you try to pick it up, you can still get it to spawn again later on in the same game.
The other screwy thing is how fast 2Dev does his runs. If I was on Nightmare!, I'd take my sweet time, figuring out how to do each move carefully. How can he manage a game that fast and get something so specialized?
That's just 2Dev being 2Dev. I don't think he knows how to play the game slowly. :)
Also, Tormouse got the Berserker Armor to spawn in the Mortuary, but he didn't get the DS in the UC. This shows that the Berserker Armor isn't completely connected to the spawning of the DS. Tormouse did a N! run on AoMc, not quite damageless, but he only got 56 points of damage. He also took a day to finish the run, so speed isn't necessary to spawn at least the Berserker Armor. He also did a Cateye build, so this could be why he didn't get the DS, if its spawning is related to your build at all. But Tormouse also entered and completed the Wall. So, if the Wall does correlate to the spawning of the DS, he may have blown it right there.
I actually did get the DS to spawn in UC in that game; it just didn't show up in the mortem. (DS never shows up in the mortem unless you successfully pick it up) As far as I can tell, the DS and BA spawn if you play a mostly damageless Nightmare! game. (It doesn't seem to appear if you do it in AoOC, though; I tried) And you may be right about the Cateye build hindering my ability to pick it up. It would not surprise me at all if being berserk is one of the conditions for picking it up, but I know for sure that it is not the only condition. You need something more than simply being berserk, something that people keep saying is simple, but I haven't figured out what that is. Whatever it is, it's something that you can do in any game, regardless of build or what special levels you play. (as far as I can tell)
Also, since when am I a mouse? ;)