Board Thread:Lore Discussion/@comment-151.225.142.234-20140517150540/@comment-16047389-20140730212755

The Last Dragonborn could not have beaten Mercer Frey. Not before becoming a Nightingale. Mercer Frey wielded the power of the Skeleton Key, and had 25 years to master its ability to unlock one's full potential. When you finally battle him, he is capable of unleashing all three powers of the Nightingales against you repeatedly. He was also capable of unleashing the final wave of magic on his death, to flood the entire ruin. The only reason the Last Dragonborn is capable of beating him at that point is specifically because they had Nocturnal's direct blessing on their side. THAT is why Karliah wanted to have both the Dragonborn and Brynjolf raised to Nightingale status, as well as to restore her own status.

How do you think Mercer was able to know that Karliah was waiting for him? Or how he managed to get all of the treasure out of the Guild's vault without being noticed? Or how he always seemed to be just one step ahead up until the final encounter? It's because all the luck that was supposed to be flowing to the Thieve's Guild (the Dragonborn included), was waning, and his own luck and potential was unlocked by the Skeleton Key. Nightingales operate as the direct agents of Nocturnal, and thus have a blessing that transcends the luck that flows from Evergloam, and is the only force that can match the Skeleton Key.

It's not a matter of being physically or mentally superior (not that Mercer wasn't strong or cunning). It was a matter of him having near limitless power, and the Dragonborn not. Your character can be level 150 or whatever, but from a lore perspective, up until you had Nocturnal's sayso, you would have been no match for Mercer, who had all the powers of the Nightingales, the Skeleton Key, and 25 years of experience using it.