Lichdragon Fortissax is an optional boss in Elden Ring, found in the Deeproot Depths. To get to Lichdragon Fortissax, you must first defeat the boss at the bottom of the ravine in the Deeproot Depths, then interact with the Deathroot found there.
Once you have interacted with the Deathroot, you will be transported to a new area where you will fight Lichdragon Fortissax. This boss is a powerful foe, so be sure to come prepared.