ceremony put together an *awesome* guide to dominion and how it spreads. You can find it here:
http://www.maladjustite.com/dominion.pdf
You can think of Mictlan temples as being like any other, except that it takes blood sacrifices to activate them. Since temples can spread dominion to nearby provinces, "spread dominion over the world" does make sense. In your test, your god was spreading dominion, which is the only "free" domionion spread Mictlan can get.