This object is a random drop in Normal Difficulty only from Hades (Drop Rate: 8 - 10%) , the main boss of the game at the end of the game, and so it'll be a while before you can reach the Secret Passage -- at least on the "normal" difficulty setting. The Overlord will stay with you and work on the other difficulty settings, and so once you've acquired it, you'll always be able to open the gate.
Aquiring this item and use it on Overlord's Gate in Coastal Asomata , opens the Secret Passage , which is an Easter Egg in the game. If the Passage has been cleared, you will found an enormous Majestic Chest at the end. It has a high chance to drop unique items (in Legendary, the avarage Drop Rate of an unique item: 20 - 35%)