Soooo, for those who have no idea what this picture is about: Bedlam, that blackness there, is what happens when you overlap the planes of Fae, Oblivion and Hell. (So I was told.) It's a sentient plane that's seeking out the lights that left it. It only awoke when adventurers went into it to save an ally...who then became the Queen of Fae.
In game, it's always been portrayed as black tarps with purple glowsticks, so it's "inky blackness that glows an eerie purple." To me, that's not quite right. More like, the blackness has the faintest edge of purple to it, but only when masses of it are overlapping, like an outstretched shape against more darkness.
So have some hapless, nameless adventurers being scared mindless.