I remember that Strahd flooded, I think Berez is what it's called, presumably with his power. I thought it would be cool and fitting that he would have some "lair actions" in Barovia. After all, he is the ancient and the land.

Some ideas I had:

It would be a power that would either recharge on a 6 (from d6) every hour, or perhaps could do one of these per day.

Teleportation: Include a tree at Castle Ravenloft. When someone carves "He is the ancient, he is the land", Strahd gains some awareness of that area. He can step through the tree at the castle and show up at the other tree. Alternatives could be using bones and hid bone chamber.

Weather: he could influence the weather on command.

Moon: he could cause an eclipse or full moon on command.

Plagues: He has the ability to create diseases. Perhaps using some zombies as a vector for the Plagues.

Nature: Making trees move or attack players, perhaps uprooting them and making them come to life.

The soulless: since the soulless people are present by Strahds will, he could cause them to act on his will. They are an extension of himself.

Mists: Strahd is able to manipulate the mists. I'm thinking he could use then to obstruct vision, make mist monsters, maybe other things.

Thoughts?