How can I do a moving gas cloud

Hi,

I wonder if you can give me suggestions on how can I implement a gameplay system in where the enemy is a killing gas cloud moving in a city an progressively covering it all in a very organic way, I mean going through the streets like a fluid will flow in a flood. My map is huge and have lot of high spots in where it is supposed i will see how the gas cloud is approaching. My first though was spawn bunch of particles in the map an toggle visibility depending on some moving logic for the cloud, but it is not the best way because it will be very expensive. I can’t just think in a good approach so I hope you guys can throw me some ideas. Thank you!

You could probably get away with a low density particle field controlling volumetric fog, there’s an image example of that here Volumetric Fog | Unreal Engine Documentation