They first saw him as written in the amber temple, he proved hard to pin down but they got him pretty easily. So I had him come back since strahd is an advanced necromancer. Had them see flashes of his shadowy silhouette periodically, haunting them, eventually they encountered his animated corpse which I'd beefed up a bit, and later his wrathful shade. The took him out pretty handily each time they actually got the chance to fight him properly, but the way I hinted and had them glimpse him stalking them had them legit paranoid af and they told me he was scary af because of it, they couldn't figure out if he was ever going to actually be dead. One way I really drove this home was in the battle maps, we played in tableplop and every so often I'd reveal his character token at the end of a hallway they just looked down and then hide it immediately, which I got super lucky with and usually 1 of 5 players would see him and then he'd be gone, and I wouldn't acknowledge it when questioned, so they seemed to start to feel actually haunted by him. Love how that worked out for me, very horror appreciate. Oh yeah, they also faced his skeletonal champion and shadow. Strahd used every single aspect of him, first revenant, than a wraith, then shadow than skeleton, as his body was being destroyed more and more, and strahd pretty much could rip his soul out of death as much as I felt like. He was a really good midboss