It forms an adjective. Danger by itself is a noun, if you add the suffix -ous, it makes it an adjective (A dangerous man). The adverb would be if you added -ous + -ly, and make it - dangerously (He drives dangerously). There is no verb "danger" or using any of its derivatives, the only way to make a verb out of danger is to add a prefix -en, and make it - endanger.
