https://forum.square-enix.com/ffxiv/threads/461853

Area and coordinates: The Porta Decumana (world area reached by using the passage next to The Praetorium's entrance - not the trial!)

1. Accept the quest "The Ultimate Weapon" after Patch 6.1
2. Reach the quest step "Confront Lahabrea on the Porta Decumana."
3. Speak with Lahabrea to start the solo duty

average item level 42 or above is necessary to proceed