Decorated or adorned with a diadem (a crown or ornamental headpiece); crowned.
From 'be-' (prefix) + 'diademed' (past participle of 'diadem'). This is a poetic/literary formation showing how 'be-' was used with adjectives to intensify or emphasize them.
This is the kind of word that poets like Edmund Spenser or John Keats would have used to create elaborate descriptions of royalty or divine beings—it's absolutely theatrical!
Complete word intelligence in one call. Free tier — 50 lookups/day.