Throughout the series, we learn through subtle dialogue and item descriptions that the undead were treated with absolute disdain and outright hatred.
Allfather Lloyd and the iron king are said to hunt undead. Lloyd probably did it for popularity, perhaps like a witch hunter, and the iron king did it for sport, as evident by the huntsman's copse.
Undead were tortured mercilessly. As evident by the undead purgatory in DS2, and the undead settlement in DS3. still living bodies are crammed into small cages, at least 5 at a time. bagged corpses hang from trees, victims of "the wheel" are strewn about.
Undead were also imprisoned. Undead asylum, Irythyll dungeon, Lost bastille, Greirat and Seigward, Lautrec, Creighton….
Think about how we start all 3 games. From a lowly prisoner of a forgotten madhouse, to a wanderer in search of god knows what, to a corpse already buried an a half-assed coffin.
So it is generally accepted lore that undead were treated with persecution before the events of all 3 games.
But what about slave knights?
It is said that they fought in the most pitiful of battles, used as cannon fodder, against overwhelming odds. They could die any number of times, but they'd always come back, never relieved of duty. Poor guys.
They are probably the type of knights who would scavenge their weapons from the battlefield, picking up the strongest looking branch, or maybe a broken chipped rusted greatsword that was probably lying in the mud.
They must not have been common. We only encounter 1 out of all 3 games. I wonder what type of king or duke would actually use a slave knight. I think most people in DS3 would agree that it is easier to just lock them up and ignore the problem. But to actually give them a sword, and tell them to fight for you…. well…. That's interesting.
I wish there were more. Like Heide knights of DS2, or black knights of DS1. That would be bloody sick, encountering them at random places… Yeah. Just some shower thoughts.