I talked about it here, though the other question was about how deadly it was and I argued that it wasn't that bad. I quoted a lot of the contemporary sources though, so you can get a sense of what they thought it was. Basically, there was a cloud of some sort that lasted for eighteen months in 536/7 and this obviously wasn't great for crops, but I still think that the plague was far more devastating and that it should be seen as the main reason for the demographic/economic decline of this period. The sources I looked at were all far more concerned about the plague rather than the mysterious cloud after all. As for the cause, I can't really say as I am not a scientist, but I have read articles (and I linked to them in my previous answer) suggesting that it was either due to a volcanic eruption or a meteorite impact.