dwarf-fortress
Using Embark Everywhere is there a way to wipe out all other civilizations and have dorfs take over the world?
I hate those damned tree hugging elves, and those sick necrophiliac necromancer scum, and those dirty stinking goblins, and…
Edit:
Oh, I don't want to remove the sap suckers and the corpse abusers.
I want to murder them all, permanently, in the most brutal possible way.
(Is that wrong?)
The world is where you create new games, you can play many games in a single world.
You may want to create a new world if you are toying with world creation parameters:
http://df.magmawiki.com/index.php/DF2010:Worldgen_samples
By altering these parameters, you could generate a world made only of ice and magma, where you are under constant siege by Goblin hordes. If you're finding the game too easy on standard settings...
So from what I gather is that you want to create a specific map or decrease the temperature on an already created map. I don't know the latter but I can help with the former:
Best Answer
As of the current version of Dwarf Fortress, nothing happens in the world post-world gen.
This means (among other things), that civilizations cannot go extinct, animals won't repopulate, and towns will not change "ownership".
While Toady One (DF's developer) has expressed an interest in the ability to send your dwarven military out to assist in wars, battles, etc, nothing of the sort is currently achievable in-game.
So, sure, you can kill everything wherever you embark... but they won't acknowledge the fact that you've conquered them.
If you're more interested in a world without elves and necromancers, your better bet would be to tweak the world-gen parameters instead.