You can start this quest by talking to Lumbridge sage he well say that there is a dragon lair on Lumbridge he wants you to defeat a dark dragon for his heart now until you see a cavern with rocks blocking it use your pickaxe to mine the rocks (level 88 Mining needed). Go through the cavern you have discovered the dragon lair! now you see a door blocking the way (Level 68 Attack needed!) Destroy the door, now fight the Water dragons. Now Poison dragons will appear (you need 57 Slayer to kill them). They are poisonous, so bring antipoisons! After that, you enter the lair. Now you fight the Dark Dragon! After that you get its heart. Now give it to the sage. Quest Complete!