Moja is the familiar of the Mehse Vienna in Knights in the Nightmare. It is suggested by the Tome of Lost Souls description that its main purpose is to provide the young Westkin with a means of communication.
Little is known about Moja's nature, but it can be inferred from Vienna's words that the bond between the two goes beyond that of a master and a pet bird. For instance, Moja appears to have an insight into the young witch's mind, often broadcasting her thoughts and reading her feelings. Furthermore, a fraction of Vienna's power seems to be contained within the bird and amplified by it in return. This latter practice, however, can prove fatal to both parts of the pact, as drawing on the pact seems to take a heavy toll on the vital force (compare to Vienna saying "I don't care if we both die... I need strength!")
Usually, Moja is inseparable from its little mistress. On rare occasions, however, it can be seen on its own doing simple errands, such as acting as a scout for the Westkin when the Wisp makes its approach. It is last seen joining Vienna in her desperate attempt at revenge, and is killed by the Wisp's knights along her.