Great Soldier of the Goblin War

Head to Grass Fields and speak to Dealos at (5,11).
He will tell you that the Goblins have declared war on all Aislings and that they have gathered nearby.
When he asks if you would like to join the battle click "Yes".

Head north and into the Grass Battle Field.
You will need to kill 2 of each type of Goblin:

Once you think you have killed enough you can return to Dealos to check.

You will receive the Legend mark: "Great Soldier of the Goblin War".
If you are a Warrior, Monk or Rogue you will also receive your master helmet;

War Helmet

Sun Bae Nim Band

Quests

Great Soldier of the Goblin War