This dungeon - used in the Legends Quest - is located in the Kharazi Jungle on the southern part of Karamja. To get into the jungle you will need to have at least started the quest and have a machete and an axe with you. The entrance to these caves is in the western part of the jungle which can be accessed by searching one of the three rocks (level 50 Agility required). The entrance brings you near the octagonal flames (Picture).
To get through this dungeon you will need a Lockpick (or two), a pickaxe, an Unpowered orb, a charge orb spell and a rope. If you are still doing the quest there are many more requirements as well.
For more information on this dungeon, see the Legends Quest guide.