She has a great sense of humor in this game. So the skeleton holds open the portal to her realm, which influences luck. Sooner or later, a thief shows up and steals the skeleton key. They steal the skeleton key from other thieves who were protecting it, who now have to go and get it back. But because its missing, their attempts to get it back are prone to failure, since the one who stole it has all the luck, and the agents trying to retrieve it have suddenly very bad luck.

Its mean, but its funny. I think Nocturnal does it on purpose.

Funny how Mercer Frey was the only competent thief in the whole Guild. Nocturnal must have been laughing her ass off at Karliah's quarter-century's worth of blundering all over Tamriel.

Don't give him too much credit. Even with a magical artifact that could pretty much allow him to do anything, he couldn't find Karliah during the time she was blundering.