I had posted a question a couple of days ago asking if it would be possible for an artificial satellite/cosmic body at the L1 orbit to block out the Sun, but the logistics of that is a little too extreme. So I'm changing tacks now, and handwaving the actual blocking of the sun by different means.
In my initial inquiry, the celestial body was a Lovecraftian entity from another world and something similar will be occurring here too, but in this case, the rift between worlds will be letting out a strange black mist from the other side that floats up into the atmosphere and effectively starts blocking out sunlight and darkening the sky.
The desired effects: start with the sky getting darker, akin to what it looks like during sunset/twilight hours. As the mist spreads, I'd assume that it gets colder as well, which would lead to disruptions in the ecosystem, and psychologically affect human beings. I want the light to be cut to a level where it would hit the biopsphere at a fundamental level and cause mass extinctions around the world to all but the hardiest animals. Let's say that the mist is some kind of supercharged version of a reflective aerosol (sulfur dioxide maybe?)
So essentially, I don't want to block out the sun entirely, but drastically reduce the energy arriving to Earth to lower worldwide temperatures, permanent dark skies and ecosystem getting messed up.
What I'm looking for is - how much would the global average temperature need to be reduced to in order to achieve these effects?
EDIT: Adding some details. I want it cold enough where even sub-tropical areas start to receive snow (Algeria, Israel, El Salvador etc.) and temperatures drop to the single digits at the Equator. Waterways start freezing over, and both terrestrial and aquatic plants start to die off, creating a devastating chain of death throughout the food chain.