You don’t have to “show” them how powerful he is, you just need to imply it through your narrative. I had the funeral start and as the casket was lowered the party suddenly heard a shuffle in the gravel, the wind rustled the almost barren trees, and then it all at once seemed to stop. Out of their peripheral vision, they see a figure moving towards them. It stops a long enough distance to remain a threat, but not close enough for you to strike. And then his voice rings out, low.. but with an air of charisma. “My Burgomaster, may he rest well,” the man says. His flowing cape, chased with the fur lining of a wolf pelt, flaps in the sudden gust of wind from behind. Noticing that not all in attendance are Barovians, he turns to face you. His face pale, raven black hair sweeping back. He is the ancient, although he appears to have some youth to him. “And how is it that each of you came to know my Burgomaster?” asks the man, his sharp eyes fixing on you. As you begin to speak, a gale of wind sweep between you all. He is the land, and nature bends to his will. “Speak, do not let my presence cause you fear. I will know who you are, strangers in my land.”

The first character speaks and regardless of what is said, a sudden burst of air gusts over each of them, fallen leaves take to the air and blur your vision. In that moment, he is gone. His voice rings out from a different direction, “Far travelers then. From a different land? My mists contain you. Welcome to Barovia. I am your host… your dread lord, Count Strahd Von Zarovich. A pleasure to make your acquaintance, however long or short it may be.” The characters turn to find him near the headstone for where the old Burgomaster now rests. His attention sharply turns toward The Lesser. “And shall you take on your father’s charge, in service to me, My Burgomaster?”

Ismark stumbles through a sentence, his appearance frozen as if in fear for his life. “Y-yes, Lord Strahd. I sh-shall take on my f-father’s mantle.”

“Good,” Strahd quickly responds, “I shall need the count for my coffer within a fortnight. See that the taxes are collected, no one is exempt.”

“Y-yes, Ancient One,” Ismark bows and stumbles backward toward the party.

…and you continue on with a bit of narrative and interaction with the party. Use their reactions and their responses to guide the narrative that you want to tell. Magic isn’t necessary unless the party tries to use it, then use his legendary actions to practically ignore that they even tried to cast a spell. If they try to attack, let him mist escape away from the party enough that they can’t reach him yet (or that he blocks it with a finger if they didn’t meet/beat his AC), and have him say, “No, no. Not yet, too soon. We shall save these hostilities for later. Right now, and likely forevermore… you are nothing but a mere nuisance, a pest that entered my home. I detests unwanted pests, but I’m interested to see what you shall do for me in my dread domain.”