The Cave of Sin is a Dungeon in the zone Red Mooshade on the Continent Feiton in Lost Ark
There are tasks in the Achievements <ALT+O> tab to find the hidden stories within this dungeon. These reward Roster XP for finding them
Where to find Hidden Stories in Cave of Sin
Here is a map to all the Hidden Story locations in the Cave of Sin Dungeon. This also conveniently shows the locations of all the mokoko seeds too! Some of the mokokos require defeating enemies nearby and then destroying vines that block their access.