There's fog_volume on other games, such as L4D2 and CSGO but TF2 doesn't support those. I assume it's possible to have more than one fog controller active at once in TF2 too but i havent experimented with fog on TF2.
You can fire the SetFogController input to players if you want it to change depending on where they are - I believe a trigger_multiple with only the Clients flag checked, firing OnStartTouch !activator SetFogController <fog controller name in the param. override box>, should workUnfortunately it doesn't look like you can make fog change depending on specific areas.
However, if it's a multistage map and you want each stage to have different fog values (not changing within a single stage), then you can send inputs to the env_fog_controller entity upon the stage changing, which should produce the same effect.
You can fire the SetFogController input to players if you want it to change depending on where they are - I believe a trigger_multiple with only the Clients flag checked, firing OnStartTouch !activator SetFogController <fog controller name in the param. override box>, should work
It is clientsidewould this method be able to set the fog specifically to the !activator client side? or will it affect all players simultaneously?