The Relic Crown is a sell-only item in Generation V. It's incredibly rare and can only be sold to the billionaire in Undella Town for 300,000 Poké Dollars, making it the most valuable sold item in the series to date. It's found on the fourth floor of the Abyssal Ruins.

Trivia

The crown has the same design as the one Ghetsis crowns N with in the intro clip for Pokemon Black and White