Blackout is an adventure of the Shards series, previously published as "Deep Trouble" in Throal Adventures. It's available as a standalone Shard from DTRPG, or as part of Shards Collection Two direct from FASA.
Far beneath the Inner City of Hustane, the characters investigate a threat to the Kingdom of Throal. What they find in the darkness, however, is far more serious than anyone had expected...