If you've seen "The Thing," or any of the other movies based on the same John W. Campbell story, you might understand where this episode got its title. Harry and Murphy are quietly discussing things in his office when Morgan, Ancient Mai, and some other minor functionaries from the High Council come bursting in, running from some evil they don't understand and needing refuge. Soon the entire building is cloaked in the black fog of DEATH!
The wizarding folk discuss the situation, despite Murphy the civilian being there to hear them. Hallucinations abound, including Harry's father, trying to get them to leave the building and die in the darkness, but most of them are able to resist. Bob is sent out on a recon mission, on the theory that, being dead, he is the only one that isn't in danger of being killed. He makes it back, but just barely, and reports that the darkness is not just a barrier; they are literally in hell! They begin to suspect that there is a traitor among them (there was), but don't know who it could possibly be.
All this occurs in the early parts of the episode, so I don't think it counts as a spoiler. The bulk of the episode revolved around figuring out exactly what was going on, who were the perpetrators, and who was the traitor. And of course what to do about them.
Nice taut gripping episode with good effects and great dialog. A solid 9 out of 10.
