The dread nature of Ravenloft within you transforms your fog into a living, hungry barrier that refuses to let its prey escape and actively pursues those who run.
Whenever you cast Fog Cloud, you can choose to awaken the fog, granting the spell the following properties:
Sentient Mists: The area of the spell remains Heavily Obscured and lasts for the duration . Additionally, on each of your turns after casting the spell, you can use a Bonus Action to move the fog up to 20 feet in any direction.
Closed Borders: The perimeter of your fog acts as a realm border. When a hostile creature attempts to leave the fog's area, it must make a Wisdom saving throw against your Spell Save DC. On a failed save, the creature takes 1d6 Cold or Necrotic damage (your choice when you cast), and its remaining Speed for the turn is halved.
Hunger of the Mists: Any hostile creature that enters the fog for the first time on a turn or starts its turn there takes 1d6 Cold or Necrotic damage (your choice when you cast the spell).
Dread Scaling
When you cast Fog Cloud using a spell slot of 2nd level or higher, the spell gains the following benefits:
Radius: Increases by 5 feet per slot level above 1st (this overrides the standard Fog Cloud rule where the radius normally increases by 20 feet per slot level).
All Damage: The damage for Hunger of the Mists and Closed Borders each increases by 1d6 Cold or Necrotic damage per slot level above 1st.
Movement Speed: The distance you can move the fog with your Bonus Action increases by 5 feet for every 2 slot levels above 1st.