The Yanille agility dungeon can be accessed from two points. The first point being north of Yanille which requires you to slash a web then cross a ledge, and the second point being a staircase near the bank of Yanille which afterwards requires level 82 thieving to picklock a door.
This dungeon is known as the 'Agility Dungeon' because of the large number of agility obstacles. For more information on these, visit the agility skill guide.
The main point of interest is Salarin the Twisted. To reach him you need level 67 agility. He can only be killed with strike spells, however if you manage to kill him he may drop a key. This key opens the chest in the western part of the dungeon, which contains a total of 9 herbs including the rare herb Torstol. For more information on this herb, visit the Herblore skill guide.