Pontiff Sulyvahn is the 8th boss you will fight and the most deadly out of them all at this point if you allow him to be.
Pontiff uses a mix between magic and fire swords and can shoot out magic projectiles from time to time. When I said deadly before I did mean it and here's why, Pontiff is very fast, his movement speed can allow him to get behind you in a matter of seconds & if he hits you with a combo it is going to deal some very major damage.
The hardest part though is when he splits into two. His double or phantom in this case will mimic all the attacks and movements he does so if you get caught in a combo from both then you're most likely to die.
However, there is a way around this if you use stamina correctly. When he gets on his knees and has black aura around him don't charge him as you will get shoved back and hurt. When he is standing still and not moving if you have been careful with your stamina you can kill the phantom as it spawns by hitting the boss damaging both at the same time.