This is the adverb: in a pernicious way — harmfully, and often quietly, so the damage is easy to miss. An idea can spread perniciously through a group. A rule can perniciously favour one side while looking fair on the page. Same formal, slightly heavy tone as pernicious. You build it in the ordinary English way: pernicious + -ly.

禍害深遠地；在不知不覺中造成傷害地