This is a guide showing all of the cubes required to enter the Last Tomb. They're shown in order of availability. Should be pretty much self-explanatory. They're not that hard to find, so I encourage everyone to search for them on their own. But in case you're truly stuck, this guide should help.
In case you prefer a text guide, here you go:
Halstedom: Get 5 moths and give them to the yellow artist guy who is in a house near the white tower. The cube is in the room you unlock with the moths.
The Family Plaza: When you enter the area coming from Halstedom, go right and you'll eventually find a house you can enter. Inside is the cube, you can pick it without fighting the two pigmen (just make sure you don't accidentally talk to them).
Island: On your way to the throne, inside the hallway that was a round room with a blank stone tablet. Look down a bit, it's near a door.
Corwid Woods: When entering the woods from the Deep Woods, go left. If you come from the other entrance, aim for the eastern corner of the woods. There is an area sealed by a blue barrier. Use the Golem Hand and enter that area. The cube is to the right of the skill totem.
Endworld: There is a house with a sole person inside it. The cube is to the right of that person sitting on the table. When you have trouble finding the house, just go right when coming from the Desert.
Halstedom: Enter the bar where you started the game. Go to the first floor and talk to the minotaur. He'll arrange a fight - you against 5 people. If you win, you'll get a cube as a prize.
Cave: On the opposite side of the house where you found the cube, read the stone tablet and enter the cave. Keep exploring the cave until you find a map. Go left from the map to find a path that leads to a stone tablet. The cube is in front of it.
Desert: Coming from Endworld, go straightforward and you should find a building that has a burning fire on top of it. That's the entrance to the Last Tomb. The cube is in front of the door on the ground.
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In order to open the entrance, all the cubes must be found (below is a video showing all the...