Here is the full list of all 16 One Day : The Sun Disappeared achievements.
Defeated the Boss Ogre
Defeated the Goblin King
Defeated the Scorpion King
Defeated the Merman Boss
Defeated the King of Fiery Spirits
Defeated the King of Giants
The World Savior
Sword Collector
Axe Collector
Dagger Collector
Mace Collector
Spear Collector
Armor Collector
Helmet Collector
Boots Collector
Shield Collector