Underground Casket

Many stories start with a small change.
Those small changes have lasting consequences.
Frisk took a detour down an alley and was killed. The resulting ghost chased a ghost hunter named Johnny Ghost down into the underground as a replacement.
Johnny isn't the one who wakes up.
Jimmy Casket, world's most renowned murderer, is in the underground, and all will know his secret.

(Killing is in this, if you are squeamish, don't read.)
(This is self beta, if you see a problem, comment. Edit: Now has been given a serious clean up.)