Windurst 5-1: The Final Seal
In Windurst, speak with the gate guard NPC. You'll learn that missions are currently on hold. Head to Heaven's Tower and go upstairs to see the Star Sibyl. She'll give you the Seal of Fei'Yin.
You should gather together one full party of level 50+ (preferrably higher) and go to Fey'Yin. Of note, aside from the starting guards/nations, Mission 5-1 is identical for all nations, so you can assemble a group of people from any nation. Equip everyone in level 50 gear and below, as there is a level cap of 50 for his fight.
You'll experience a cutscene upon entering Fey'Yin. Once you're done, proceed through Fey-Yin towards (K-6) in the top-right corner. When you enter this room, you'll find a Cermet Gate. Open it and enter the Qu'Bia Arena.
Once you enter, you'll want to put on any songs, enhancing magic, food, etc. before proceeding. Here you'll be fighting Archlich Taber'quoan and a small handful of other skeletons. Archlich Taber'quoan is the hardest, and the only one you need to actually defeat to complete this fight. He's a Black Mage (BLM) so be aware of his ancient magic, Sleepga II and two-hour.
Once he's been defeated, return to the Star Sibyl.
Next: Windurst Mission 5-2