Civilizing the "Uncivilized" is a major element of the game, but currently the Monstrous civilizations cannot change civilization tiers - it wasn't something we felt we could do justice to in the design time we had. However, you can still take over the world - certain structures will reinforce and strengthen your dark allies otherwise weak governments to make long-term occupations a possibility. This is not the most common route to go - more likely you will be leveraging each of your allies strengths and allowing them to fall on their weaknesses as well, relying on the multiple threats to win in the end as opposed to championing a single nation like you would in a more traditional strategy.
If you have sufficiently corrupted a civilized nation, it does become feasible to create a world-spanning empire but it is incredibly difficult in some scenarios. For example, in the North Burns you have only two nations that could possibly take on this task , Arden and Cylaria (MAYBE New Caledon but I've never seen that happen) - Cylaria would be a strong ally as it borders nations you can easily control, allowing you to naturally combat Arden - managing a victory here would almost assure you the game. However, Cylarian culture is heavily paranoid of foreign influence, shadowy infiltration, and in particular orcs. Even if you were to corrupt the King, his Feudal Barons are a powerful check on what you can actually accomplish - and fomenting a coup with your agents would be met with hostility from the aristocracy. Put simply, the situation isn't amenable to this approach (but you can do it, with varying levels of success based on what build we're on - even a small change in the AI or importance to a statistic can tip the scales).
Next door you have Arden, a cutthroat nation of mercenaries and profiteer slave-drivers ruled by a military dictator. Yes, obviously, this is a much easier nation to bring over to your side. However, it doesn't have the kind of cultural bonds that Cylaria does - maybe you do coup the King, install your own Agent and create a Dark Empire. Revolution would almost certainly follow, with several regions breaking away, and then you are threatend by Cylaria desperate to reclaim their ancestral land, New Caledon who loath your breakaway status, and possibly even Toln or Aventura, both strong enough to combat a weakened state, even The Theocracy of Olm on your southern border may join in if they believe in that your agent is driven by an Old One. This is definitely a "fun" route to take, but if you aren't properly prepared with other mechanisms it may be a fruitless exercise.
Of course, all this really does shift with each AI build we do - we also are constantly building in new Actions, some of which can greatly push a nation in a given direction.