Vorgel the Sinner can be found past the Belfry Gargoyles, underneath the Upper Ramparts bonfire. He is surrounded by several Stray Hounds and invades the player when they reach the bottom of the ladder.
Vorgel is located in a secluded area, and only invades the player when he reaches this small area. It seems as if he's guarding the Bastille Key, which may lead to the assumption that he has sinned against the prison and released the prisoners.
Vorgel himself is not a major threat; the hounds that are present with him are the real threat. The small area means that the player does not have much room to move around and risks being swarmed by the hounds and Vorgel. Death is almost certain if the player is trapped in a corner. To make matters worse, Vorgel's attacks have pretty long range due to his spear and can easily finish off players that are trying to heal.
He will not follow the player up the ladder, meaning the player cannot bait him up the ladder to fight him one-on-one. However, this does mean that the player can finish him off or the hounds from above the ladder with ranged attacks, but this can be hard to achieve due to the ladder blocking the player's view.
Another alternative is to use the Strong Attack of a Halberd; the spin attack will take out most, if not all, the hounds if they are grouped together and inflict damage on Vorgel himself if he is caught in the attack. Once all the hounds are dead, Vorgel can be easily disposed off.
By far the easiest way is to use a Seed of a Tree of Giants to turn the hounds against him.