In Elden Ring, players will come across a few NPCs that are essential for reaching certain boss fights. Fia is one such NPC, and players will get a chance to battle the Lichdragon Fortisaxx if they complete her questline.
The Lichdragon Fortisaxx is located at the Prince of Death's Throne Site of Grace at Deeproot Depths. This boss is only available in Fia's questline, and the closest Site of Grace available is Across the Roots. Players will have to interact with Fia and enter her Deathbed Dream to fight with this legendary Dragon.
After reaching Across the Roots Site of Grace in Deeproot Depths, head north, and you will be invaded by three of Fia's champions. Defeat them to interact with Fia, and ask her to hold you one more time for the Baldachin's Blessing. She will ask players to acquire the Cursemark of Death as part of her quest.
Acquire the item and bring it back to Fia. Interact with her, save the game, and return to the same location to find Fia asleep. This is where players can enter Fia's Deathbed Dream to fight the Lichdragon Fortisaxx.
Deeproot Depths is an underground zone that can be reached via the Great Waterfall Basin Site of Grace located at Nokron, Eternal City. Defeat General Radahn to unlock Nokron, and make your way toward the Great Waterfall Basin Site of Grace. Players can enter the coffin at the edge of the waterfall to wake up at Deeproot Depths after a cutscene.
Wondering how to get to the Forbidden Lands? Check out What to do after defeating Morgott the Omen King in Elden Ring? on Pro Game Guides.