From Dust Game Guide

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                                  | Origins |                             [S13]
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This is as close to a sandbox level as you're going to get in this game. 
You can lose this mission, but it'll be by your own hand if you do. You are 
given true god powers in this.

Press A on the earth to erect a totem, then press A again to send your men to
it. This gives you the breath of earth power, which lets you create earth 
from nothing. Press Left on the D-Pad to activate this power. The Left Trigger
erases earth and the Right Trigger creates. Press Y to make a small hill 
come out of the water. Hold Y to make a huge mountain of earth come up. Press
B to make a crater, and hold B to make a big crater. Once you've got a nice
stable piece of land far enough from your original totem, press A on it to 
place the next totem, and send your men to it. Once they capture it, you get
the breath of lava power.

Press Up on the D-Pad to activate this power. Pressing the Right Trigger 
creates lava, Left Trigger erases it. Press Y to make a little volcano, hold 
Y to make a big volcano. Pressing B eliminates the lava source, but you have
to be hovering right over it to eliminate it. Holding B will cause whatever 
you're hovering over to sink. A lot. Make your next totem to get the breath
of water.

Press Right on the D-Pad to activate this power. Press the Right Trigger to 
create water, Left Trigger to erase it. Press Y to make a source, and hold Y 
to create a tsunami. This can wipe your villages out, so be careful. Press B 
to eliminate a source, and hold B to use the power of evaporation. Your next 
totem gives you the breath of plants. Press Down on the D-Pad to activate it.
Right Trigger creates vegetation, Left Trigger erases it. Pressing the Y 
button gives you either a fire tree, a water tree, or an explosive tree. Keep
pressing it to cycle through them. Holding the Y button causes a large area to
be immediately covered in vegetation. Press B to erase trees, and hold B to 
erase vegetation from a large area.

After a while, or after you build enough land, I'm not sure which, a cutscene
appears. After this, you can hold A to create a portal. Make it close to one
of your totems, because after you create it, the land will start to collapse.
You'll need to build up earth to get your people to the portal. Once they've 
arrived, press A to finish the game.