Found in Catacombs of Carthus.
Greatsword: Don't remember it very well, will come back to this later.
Curved Sword + Shield: Aside from pretty regular blocking and attacking behavior they will often use a move where they roll towards you about 3 or 4 times, dealing slight damage as they roll. Employs Carthus Milkring effect.
Two Carthus Shotels: The most troublesome of all.
They attack with two swords at once, spinning and hitting you in rapid succession, causing blood loss if you are hit to much.
Some of their dodge moves will include them throwing 5 Kukri.
Often move sidewards once at close to mid range, dodging most projectile attacks. Slow casting spells will be interrupted or simply miss most of the time.
One should definitely avoid using very light weapons. Longsword or comparable should be two-handed. Once you hit them they shouldn't manage to dodge away or counterattack. If you do not have enough stamina, try timing it so the longer stagger animation (bent over from your blow) plays.
These enemies (all Carthus swordsmen) are weak to blunt weapons. One normal two-handed attack from a Club can knock them down to the ground completely. They can then be hit, knocked down again (with some timing), or backstabbed before they can stand up. Having a good blunt weapon ready can make fighting these enemies a lot easier. On the other hand, even strong attacks with ultra greatswords cannot knock them down, unless fully winded up. Something else that also works is shield bash, or attacks from a shield while being two-handed. Shield bash drains FP, however, and without sufficient FP the shield bash can no longer knock these enemies down. With a greatshield, a normal two-handed attack is enough for a knockdown. If you can use magic, Force, Emit Force, or Boulder Heave all work great.
Almost always drops a titanite shard. Sometimes drops a large titanite shard, instead. Sharp gem drops only very rarely.