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The manual, on page 93, give the basic details:

Mictlan's home province, temples, and prophet do not spread dominion. Mictlan's god spreads dominion, but is only half as effective as other gods.

However, if you left-click on your Temple with non-Mictlan, you normally see:

Conversion Rates
God 30
Prophet 10
Home Province 10
Temples 10 (Can be 10 * number of temples)
Victory Points 0 (Can be 5 * number of victory points)

For Mictlan you see:

Conversion Rates
God 10
Prophet 0
Home Province 0
Temples 0 (temples do not spread dominion)
Victory Points 0 (victory points do not spread dominion)

So, Mictlan's God's dominion spread may be 1/3rd as effective, not 1/2, as the manual says.

Also, I ran a test. 2 nations, one Agartha, one Mictlan. Both with a Dom10 god. Mictlan blood sacrificing 3 slaves every month at the temple.

That should be 3 temple checks for Mictlan at Dom10, plus either one automatic increase or one temple check for the god; versus one automatic increase and 5 temples checks for Agartha (5 = god+god+prophet+home+temple).

But Mictlan's dominion spread equaled Agartha's. This was on Glory of the Gods, up to 40 turns.

Conclusion: I don't know the exact details of how it works, Mictlan should have ended up with less dominion than Agartha. But blood sacrificing is a really great way to spread dominion!

Edit: Repeated the test, this time with Dom3 god for both nations. Same result:

Mictlan sacrificing 3 slaves per month with an awake god and alive prophet, equals any other nation with equal dom pretender, one temple, awake god and alive prophet.