there is actually 2 knights by the bonfire :Demon Ruins.
once you spawn, go up and kill the 3 Smoldering Ghru in the adjacent room. on one path you will find rats.The other path leads you to a black knight with a great axe fighting a Ghru Grunt. This path should be cleared first as it is easier to get to. you can also find a black knight sward here.
At the end of the rat path there is a black knight with a greatsword. be careful as Knight Slayer Tsorig hangs out bellow you near the lava (can be bated into it for easy clear)