@Vaftrudner - then everything is a potential spoiler, from your first caravan to caverns, to magma, to moods, artifacts, and, well, everything.
My point is that it's only a "spoiler" because some people want it to be SUPAR SEKRET because, well, frankly they place an inordinate value on shock. The status of demons as "spoiler material" is essentially arbitrary. It's no more or less shocking, really, than your first siege and the moment of mass anticipation is gone. That moment was back in March when everyone who was already familiar with 40d was salivating over finding all the new little things in the new version.
At this point a newcomer is only going to know something is a spoiler because other people are telling them they're supposed to think it's a spoiler. It's not a book, the reveal doesn't have self-evident weight in the same way the killer's identity does, and in the flow of the game it's just as "there" as ambushes and irritable wildlife. I'm not saying the spoilering was and always is bad, but I think the moment is kinda long gone now that there's multiple projects per day working on colonizing hell. Seriously, some days the front page just looks like *spoiler* project *spoiler* *spoiler.*
And, at the end of the day, even knowing there's demons down there still isn't going to do much to dampen the pants-wetting terror from the dialogue box that pops up because the presentation is what makes it so good, not the raw surprise.