The Old Monk is nothing more than a black phantom summoned by the original Old Monk, and is being controlled by the golden robes.
If the player is not invaded, or is playing offline, the Old Monk's weapons are a pair of Claws. He will be wearing the Gloom Set and the Yellow Head Collar, and at times will take out an Insanity Catalyst and cast Homing Soul Arrow (for up to 5 orbs). However, what makes this boss unique is that if you invade someone in or are invaded in 3-3, the invader will become the Old Monk instead. If you are the Old Monk and you win, you will gain your physical body as well as the Yellow Head Collar head attire.
The original Old Monk was the husband of the queen of Latria. He was banished from Latria for his depraved nature and returned with horrible demons in tow and new power emanating from his new Golden Robe. This led to the fall of Latria. 
His collar bears the description: "Head wrappings that were once the golden robe worn by the old master of Latria and eventually took control of the then withered old man. It is whispered that the robe now seeks to summon those who offer their aid, and carrying out its wishes grants them the use of these odd head wrappings. Could the golden garb be seeking out a new vessel to subjugate…?"
While it is possible that the Old Monk can be another player, and thus be completely unpredictable, there are some strategies for fighting the NPC Old Monk that will be fought if playing offline or if the player is not invaded. The NPC black phantom is very aggressive, and will often attempt to close the distance between himself and the Slayer of Demons rather quickly. The NPC Black Phantom can be stun-locked relatively easily, however it is worth being noted that he is capable of parrying the Slayer of Demons, thus caution should be taken. One viable strategy is to attempt to parry the NPC Old Monk, and riposte for a large amount of damage. Another strategy is to try and use the many chairs scattering the boss room to impede the NPC Old Monk's mobility, however this will likely bait him into casting more Homing Soul Arrows. Using a large weapon which knocks opponents down such as the Meat Cleaver or the Dragon Bone Smasher is very effective, as one can keep knocking the Old Monk down, allowing for an easy victory.
- The Old Monk is probably a reference to the King in Yellow, a book of short story wrote by Robert W. Chambers. It describes the King in Yellow and his Yellow Sign, an entity who can do mind control or possession, he gets integrated into the Cthulhu's mytho as Hastur.