Bravely Default has crossover enemies from Final Fantasy: The 4 Heroes of Light. In addition to Beelzebub and Lucifer, Asmodeus will venture into Bravely Default: For the Sequel too. Asmodeus is originally in disguise when the 4 Heroes of Light warriors meet him, but in Bravely Default you can see his serpentine form right away.
Asmodeus will venture online between December 10 to 12. Square Enix recommends players be level 70 before fighting him, which means this guy is out.
Mammon, another Final Fantasy: The 4 Heroes of Light boss, will appear in Bravely Default between December 13 and 16. Square Enix says you should be ready to face Mammon at level 25.