To me, your first supposition makes more sense.
slip into something (a phrasal verb)-to gradually start to experience an unpleasant or difficult situation (the second dark age -in your case). ON-is just a preposition for "track"-on an unsurprising track.
To simplify the whole thing, I'd say: Civilization faced its second dark age on the track of bloodshed which brought unexpectedly huge amount of deaths/suffering/horror.( everyone was aware that the new age would bring new bloodshed, yet nobody expected it (maybe the world war) to start that quickly and bring so much horror and suffering.)
*The author is also not surprised that the new dark age civilization was experiencing brought blood and deaths, but it's the amount of horror (wars/bloodshed/suffering etc) and, possibly, how quickly new wars or bloodshed started, that was shocking and unpredictable (this idea is conveyed figuratively by saying "with a speed that could not have been foreseen by even the most pessimistic futurist").
I hope that'll help.