How would you run a vampiric mist inside a fog cloud?

[FabulousFizban]

i suppose this is the same as asking how to run an invisible stalker, kind of.

[Safety Sword]

i suppose this is the same as asking how to run an invisible stalker, kind of.

Automatic surprise, advantage on all attacks for being unseen and disadvantage on all attacks against due to invisibility...

I mean, they're just the rules... not sure what you're actually asking...

[JackPhoenix]

Automatic surprise, advantage on all attacks for being unseen and disadvantage on all attacks against due to invisibility...

Surprise would depend on the mist's stealth check vs the group's passive perception, being unseen doesn't not make it undetectable (the description mentions it reeks, and there's no mention of it being silent). Same would apply to Invisible Stalker: it can Hide without needing other sources of obscurement (unless the opponent has ability to see invisible creatures), but it's presence and even position is still detectable unless it uses stealth.

And advantage and disadvantage go both ways, the mist lacks blindsight, so it would be just as blind in the fog as anything it fights. Advantage and disadvantage negate each other, so you would be able to attack it normally, and it wouldn't get advantage itself.

[Safety Sword]

Surprise would depend on the mist's stealth check vs the group's passive perception, being unseen doesn't not make it undetectable (the description mentions it reeks, and there's no mention of it being silent). Same would apply to Invisible Stalker: it can Hide without needing other sources of obscurement (unless the opponent has ability to see invisible creatures), but it's presence and even position is still detectable unless it uses stealth.

And advantage and disadvantage go both ways, the mist lacks blindsight, so it would be just as blind in the fog as anything it fights. Advantage and disadvantage negate each other, so you would be able to attack it normally, and it wouldn't get advantage itself.

I take your points. I would just run with the party knowing something is up with the smell, but unless they attack smells regularly, I would probably assume the vampiric mist is hiding in the mist... why else would it be in there?

And as for blindsight, that's a weird one honestly. I would has assumed blindsight to be honest, so you're right, except for the fact I would give it blindsight to make it work the way I thought it did.