Creating Apocalyptic Darkness?

[Jastermereel]

I'm trying to develop the BBEG in an upcoming campaign as a Duergar arms dealer seeking to take over the surface. Instead of having him use magic to modify their perfect race to not be troubled by sunlight, I'm trying to find a way to have him "terraform" the surface by way of creating clouds over a large area of the world.

I'd like to be able to do it within RAW, so is there any way to do this with an odd combination of spells or creatures or should I give in and use a DM's right to just throw a Macguffin artifact in there?

Are there any summonable creatures that create clouds or smoke that could be substituted into the Vivacious Owl Dirty Positive Bomb? A warlock with an OCD Geas on him? A neverending darkmantle swarm? Anything?

[J.Gellert]

I just saw this in another topic.

Granted, it's an epic spell, but...

Also remember: If it's just the Big Bad doing it, he can use the Power of Plot. Doesn't have to be a 'real' spell.