Lost Logia are the Bureau's classification for dangerous artifacts, typically from the lost colony of Al Hazred. That colony developed amazingly powerful magical items, some as weapons, some for other purposes. But they grew too powerful, and an accident destroyed the colony. Its artifacts were scattered to many worlds. It's part of the Bureau's mission to contain and regulate Lost Logia. The Book of Darkness, which the Wolkenritter possess and are managed by, is an extremely powerful Lost Logia.