Fog demons can live for several millennia's. The reason they don't a lot is because their reputation.

Fog demons in their real form are on all four limbs. In their real form they can be 5-8ft high. On their back legs they are 13-16ft tall.

Because of their loose body they can fit pretty much anywhere. They have 3-8ft long tails and about half a foot long spine ridges on their back.

They weigh about 5-20 pounds.

Fog demons tend to be evil, that is not saying there isn't good any good fog demons.

Fog demons get plus 2 dexterity and 1 wisdom.

A fog demon has a known weakness, they hate water (vulnerability to acid damage.)

Fog demons fog like structure let's them change color and change form.

At different levels shows what you can turn into. Any creatures CR lower or equal to your level you can turn into.

At level 1 you can start with only beasts and creatures you seen. When you shift it last until death, spell, or wanting to change form. You can only turn 3 times a day resets every long rests.

Level 6 you can turn to creatures you have heard of and have a pretty good image of what it looks like. Turn 4 times a day, rests every long rest.

At level 10 you can turn into friends, dragons and aberrations. 6 turns a day resets every long rest.

Level 16 you can turn into any creature seen or not  but you need a good image of what it looks like. Same turns at 10th level.

At 17th level you can turn unlimited amount of times.

Because of their long tail they can use it as a weapon.

Its range is the length of their tail and does 2d4 plus dex modifier and proficiency.