This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
龒 combines 龍 (dragon) and 示 (altar, show). The exact historical development is uncertain, but it likely relates to a dragon-related ritual or divine concept.
A dragon 龍 rising above an altar 示, but what it means is unclear. Imagine an ancient, forgotten ritual involving a dragon spirit.
For リュウ, picture a dragon (龍) letting out a long, echoing roar: ryuu -> リュウ, as it hovers over the altar.