The Fallen Knights were the ones who took place in The Battle of Balance. Unfortunately they all have fallen (Obviously) to the wrath of the lights. But due to Dave Jr.'s time anomalies, the Void of Time has been opened and they were let through into the future.
The only way to summon these knights are by unstable shards based off of their ores. But if you finished the game, you can summon really buffed vesions of these by taking the quest by Phantox.
Ranked from weakest to strongest: