Complex sentence: There was a long, narrow staircase which led to the basement, walls and stairs painted black which, due to the high probability of someone pitching headfirst down the stairs in darkness, was dubbed "The Descent into Hell" by clubgoers.
Split into simpler sentences: There was a long, narrow staircase which led to the basement, walls and stairs painted black. Due to its high probability of someone pitching headfirst down the stairs in darkness, it was dubbed "The Descent into Hell" by clubgoers.