Sword of Truth/WMG

The Keeper of the Underworld is Nyarlathotep

 * The Keeper is never fully described, much like most Lovecraftian entities. He also, like Nyarlathotep, wants to spread chaos throughout the universe.

The Sword of Truth RetCons history, and does nothing else

 * This one seemed so obvious to me it almost didn't seem worth listing. The Sword does not imbue its holder with righteous rage. It has no power to avoid hurting an undeserving target. Its explicit power is to make its wielder's perceptions of the world be correct, and become correct in the past.
 * This first became obvious to me when Richard's brother revealed himself to be a villain. He wasn't a villain, and he wasn't the serial killer - until Richard, in a fit of jealousy, wanted him to deserve to die. And so the character who'd been revealed to be said serial killer disappeared from the story (and possibly, existence), and Richard's brother was transformed into the kind of person who would and did commit all of the killer's crimes. By the time he was holding the Sword, it was too late; he was already rewritten by its influence. In subsequent cases it's equally obvious that whether somebody is a good person is entirely defined by how much Richard happens to like that person.
 * Likewise, every thing Richard's done during the story is perfectly justified by the fact that he did it, making it the right thing to have done at the time. As the books are written mostly using the original timeline rather than the eventual one, they simply appear to be dubious. We have Protagonist-Centered Morality as the only option.
 * Note that this perfectly combines the average reader's perceptions of the description of the Sword with the philosophical doctrine the author eventually preaches through Richard. At its simplest, morality is defined in this doctrine by the effectiveness of the actions a consciousness takes to preserve itself. Suicide is innately amoral (outside any conception of morality), and actions which impede personal survival are immoral to the (very) arguable degree that they do so (thus the issues the books have with pacifism, mercy and charity). So an artifact which causes your decision in a situation to become your ideal option in that situation is by definition the essence of Goodness and Truth, the capitalized virtues. By allowing Richard to begin the series with concepts of the virtues more in line with those of the average reader, readers simply don't notice the discrepancy at first.