User blog comment:Omega natsu2/Chapter 364: Tartarus Arc, Part 1: Immorality and Sinners, Predictions./@comment-5303512-20131211235214/@comment-1359322-20131212053339

There's a possibility that Mashima wrote Tartaros Arc as the title because the next 7-8 chapters focuses more on the Tartaros side than the FT or Legal Guilds side.