Update: he also drops a Knight's Great Sword

Update: he also drops his Golden Great Shield as well

Drops pieces to the Leyndell Knight Set (ie; Leyndell Knight Helm, Greaves, etc)

All of his gear is all lootable, Leyndell Knight Helmet, Armor, Gauntlets and Greaves

There is also one of them at the bottom of the deeper of the two lifts of the Rampart Tower area.

It is accessed by dropping down from a ledge onto a rocky outcropping which will collapse, just outside near the Rampart Tower Lost Grace.

Hug the left wall and go up the small set of stairson the platform then drop down to reach it. I suggest bypassing it to reach the said lift to open up a shortcut to reach it directly.

the black knife catacombs are guarded by a mausoleum knight, and not by this type of knight.

Their lightning bolt attack has 2 parts to it: the sword slam (which can hit you if you're in melee range) and a lighting strike that comes from the sky. If you get hit by the sword slam you're (almost?) guaranteed to get hit by the bolt. The easiest way I've found of avoiding this (at least with light-roll) has been to move/run away when they're charging and then dodge-roll as their sword strikes the ground. This should allow you to avoid the follow-up lighting bolt.

Want to hard to break his posture with a greatsword, lightning was still hell to dodge

There's a handful of them across the bridge from the ninth mt Gelmir campsite grace.