The Trail of the Sinner is the path to where the Composer resides. Neku, Beat and Shiki first travel down it at the end of the main game in order to defeat the Composer (who Neku thinks is Mr. Hanekoma) and be restored to life.
 Another Day
Once the Kindred Spirits have gathered, the Trail is unlocked as the source of Joshua's Boss Challenge. It appears that the area is the only place that retains any sort of essence from the Underground.
From Trail of the Sinner Go..