Knight King Rendal
Lore
The legendary Knight King Rendal was from the ancient kingdom of Balder that "came to ruin after a great many Undead were spawned".
A comment from the Crestfallen Merchant indicates that Knight King Rendal attempted to conquer Anor Londo and failed. It is likely that his knights followed him and failed as well, left to hollow in Lordran:
Let me give you a nibble of advice.
Don't even consider visiting Anor Londo. Not in your state.
For a century, they have tried, and failed.
The Knight King Rendal, Black Iron Tarkus, and even Logan himself.
You won't stand a chance. You'll be eaten alive.
Theories
King Rendal is Still Alive
Sources
PREV: Fina, the Goddess
I found a video by General Sam regarding a theory involving Knight King Rendal and thought someone might wanna check it out and add it to the theories page. The video itself is quite interesting and theorizes his travels to Lodran along with how he might still be alive as a member of the Darkmoon Covenant. It also gives a unique view on the idea of OnS being an illusion.
The video is called "Dark Souls : Knight King Rendal".
Thanks! I'll add it.