When playing Across the Obelisk you may come across an interesting item called the "mysterious key". However, the game does not inform you what to do with it. Instead, it is one of the items used to unlock the extremely rare "Last Piece" achievement. Here you will learn what exactly to do with it (if you already have it) or how to start a whole series of events leading to unlocking the achievement.
The secret quest in Across the Obelisk - how to complete it?
The secret quest that will allow you to unlock the "mysterious key" achievement involves a series of steps to be taken over the course of many stages of the game. In the end, you will find the powerful item "Luffy's Hat", which is the ultimate goal of the achievement. In order to accomplish this, you must complete the following steps.
- The region where you will find the first item related to the quest is The Kingdom Of Senenthia, which is the land from the first act of the game. You must go to the node located by the lake in the lower left corner of the map. However, this visit should only be made when the node is purple. If this condition is met, use the "search for anything interesting" option. In this way you will find the treasure map. Following its directions you will find some useful items, and at the very end also another map, which you will use in the next area.
- The next location to go to is The Aquarfall Marsh from the second act. In this region you need to reach the quest node, which should appear in the location marked in red. You can do this by using either of the two boats (marked in purple). After arriving at the quest node, choose the western pillar, which was marked in the treasure map you acquired earlier. This will give you the "mysterious key".
- When visiting Velkarath, enter the volcano. You will fight a challenging battle there, after which you will receive another "mysterious key".
- You will make the next (and last) step to getting the achievement only in the fourth act, in The Void. You have to go to two places (marked in red), which will lead you to an extremely challenging battle. After winning it, you will gain access to a chest with very powerful items. Among them you will also find "Luffy's Hat", which will be connected with unlocking the "Last Piece" achievement.