Echo Cave (EC) is the fourth dungeon in the game. It is located in Ancient Forest at (X: 632, Y: 163) This Main Quest requires item drops and mob kills, so be sure to have space in your backpack and to kill mobs along the way to the bosses.
You will get an automatic quest called [L40. A Sad SOS Message] when you reach level 40. This quest requires you to talk to Barriet in Land of Sighs (X:732, Y:1133).
Once you talk to him you can grab the quest [L44. Emergency: Assault Draze] You do not need an item to complete this quest, you just need to kill Draze and kill 20 Forgotten Stone Soldiers. Once you defeat Draze, turn both quests in to Adrian's Tomb near the top of the dungeon.
Adrian's Tomb will then give you a quest called [L45. Adrian and Basse] which requires you to kill Shiva, collect Shiva's Ward Dispeller, and kill 10 Flesh-Rasping Worms and 10 Lost Beasts. You will need your sprite out/space in your backpack to get the item.
Once you defeat Shiva and complete the quest turn it back in to Adrian's Tomb where you'll then get the last quest [L47. Tragic Women] which requires you to defeat Basse (the final boss in the dungeon) and collect Basse's Diary.
Once you finish [L47. Tragic Women], turn it in to Ilya Guard Captain Augusque in Ilya (X:285, Y: 135)
The objectives of this dungeon are boss kills, mobs kills, and two item drops. Be sure to remember the mob kills or you'll have to reset the dungeon to complete the quests.
Also, make sure you kill the Key-Filching Hobgoblin to get the keys required to entire the final boss room. He can be found at any of the boss areas, usually Draze or Shiva. Once you have keys, talk to Adrian's Tomb and select "Being Regretful" to go in the final boss room.
Side Note: Interestingly, you DO have to kill the final boss, but you don't have to kill a third boss in the dungeon for this Main Quest. His name is Sivalee, and he is only required to kill if you're doing the Mission Order. The title from the Mission Order is [Echo Cave Explorer] (Treasure Drop Rate +10%, Movement Speed -5%)
Extra Side Note: Echo Cave actually has a really sad story, so if storyline is your thing you'll probably enjoy its Main Quest story.