Dude, you need 4x Mutavault . They can tap for their own mana to animate and they're all creature types - that includes RAT. If it's a budget concern I get it, just making sure you were aware ;-)

Carry on!

Have you considered Ogre Slumlord ? He's kinda the dude when it comes to rats, plus boosts that devotion a little bit.

I would love to throw in mutavault, but I am worried about not being able to cast Nightveil Specter if I don't have the swamp. Maybe two or three?

Ogre Slumlord WOULD be fantastic, but I don't know what I'd remove for him. Maybe Underworld Connections?

I'd +1 Nykthos, Shrine to Nyx , +4 Mutavault , drop your swamp count to 16 for 22 total lands. I'd probably drop the Abhorrent Overlord in this deck to make room. Then consider reducing or removing Corrupt for something like Ultimate Price or Devour Flesh for better removal.

Here's my (similar) deck for comparison and ideas: Mono-Black Domination