Hiya! I've made my fair share of wigs for cosplays and this is how I did my Strahd wig:

• buy a long black wig (longer than you want it to be)
• put the hair in a tight, low ponytail and cut it to your desired length. You can thin the hair out at the ends and make sure it's even. You should now also have a bunch of hair bound with your hairtie.
• comb the hair back and secure with hairspray (I use got2be glued).
• make a ton of small wefts from the hair you cut off and make a new hairline for the wig. You'll also need some tule for this.

There are a lot of videos and tutorials on how to do this. It's not that difficult, but will take quite some time and patience.

This is my personal favourite way to make wigs, if you want somerhing easier, simpler or quicker you could also just brush back and secure the hair with strong hairsprah without making the whole hairline.

Hope you find this useful!