The fanfic is Karl Rim's "Dark Half" (sometimes titled "The Darker Half").
Synopsis from the TVTropes Fanfic Recs page:
Vampires are taking over Nerima, the authorities prove useless, so it’s up to Ranma and the rest of our heroes to stop them. Contains numerous crossovers with other 90s era animes.
The figure in the mask is named FireWater (full name Jo FireWater according to their library card, formerly Jo BlueRobe). Apparently an original character.
[ When the stranger turns we get to see his face. But he doesn't have one.
Instead, a smooth stainless metal circular mask is in the middle of his
hood, the only features on it being two small eye slits. And on his
chest seems to be the markings of four small holes ]
Stranger: Another one!
[ Ranma takes a flying leap at the stranger only to encounter a puff of mist ]
[ At the other end of the room, a swirl of mist starts forming,
and a deep voice like the whail of a ghost calls out.. ]
I am the terror that guards in the night
I am the laser that burns away darkness
I am.... FireWater!+
Character Profile: FireWater
Very little is known about this man or woman. He keeps his
identity almost completely secret and has had various of aliases. In one
past life he was known as Jo BlueRobe, and at this time had a falling out
with the Fang and some of it's core memebers such as PurpleCoat (PC to
him), GreenCoat (GC to him), and the Fang Emperess.
He calls himself a specialist in the supernatural and investigates
odd occurences, usually to combat some dark force of evil. But he and the
Fang seem to have a grudge from a long time ago. It is even speculated
that FireWater is hundreds of years old. He also apparently has some links
with Happosai, claiming to be his teacher. He often refers to the
Wandering One, some unknown cosmic entity force of good, as his mentor
although the two have barely met.
It is also speculated that he has some sort of spirit transferal
power which enables him to possess a host. But he is very lothe to do so,
since the last thing he wants to emulate is a life-stealing vampire, even
at the cost of his own life.
He seems to be a modern person who is constantly learning and
mastering new skills, to advance with the times. It is known that he has
cyber-punk training. Although he is a complete mystery, whatever his past,
his goal always seems to be a noble one.
The full extent of FireWater's powers has not been seen. He has
deep knowledge of the occult and has mastered to the full extent, a wide
variety of special martial arts techniques. His most powerful yet draining
techinique seems to be his: Winged Griffin Pounce, where he summons his
full energy into the form of an unstoppable giant night-blue Griffin image.
It is unknown if he has any magical or other powers but it is known that he
has some degree of psychic awareness and mental powers. It is known that
he is not completely human.
A tall seven foot man wearing plain blue robes and a shiny circular
facemask devoid of any features save two eyeslits. Kind of like a cross
between Lover-boy from Vampire Girl Miyu and Jason from Friday the 13th.
Unlike both, he talks.
The scene where he uses his mask to escape:
[ FireWater starts spinning his head, slowly at first and then he started
picking up speed. With a sudden snap of his neck he flings his circular
faceplate off. It spins through the air and rebounds on one of Soun's
chains and returns towards FireWater. The disc shreds through the
seal on his chest. From the angle of his hood, FireWater's face is still
unseen being covered by shadows.]
As to how I found it, after posting that I remembered reading it at the same time as Dark Raven (which I found via a Google search, remembering that the guest protagonist had a special attack which involved him yelling "Quake with fear" and chains went everywhere), and finding a site with bunches of Ranma 1/2 fanfics gzipped that looked vaguely familiar, I downloaded the entirety of the directory, uncompressed them to text files, then started searching for key phrases.