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.
The description of the Ring of Steel Protection reads:
Of the many legends surrounding the Knight
King Rendal, one of the more well-known speaks of his standing down a giant drake and slashing it to pieces.
The Silver Coin has the portrait of Knight King Rendal on its face:
Coin made of silver, with a portrait of the legendary Knight King Rendal on its face.