The Crown of Power was the crown of the Grand High Vampire. It was supposedly made out of the bones of victims from the dawn of time.
The Crown of Power was worn by the Grand High Vampire until he was staked. Boris Dracula, dressed as Justice Moroi, tried to wear the crown but it turned him to dust as he wasn't the Chosen One. It appeared in the final episode of series 2 and turned several vampires to dust as they also weren't the Chosen One.
Vlad put on the crown and it gave him lots of advanced vampire powers, to the extent where he mindwipes an entire group of people, including slayers as well as people he cares about, who a normal vampire normally couldn't mindwipe, at once. After Vlad falls to the floor unconscious, Ingrid kicks the crown away. It's unknown what happened to the crown afterwards as it doesn't appear again.