It's a video game term. It means that the features of this helm are inherent to the "Last Boss" character style, which is usually described in such words as: "evil", "terrifying", "big" etc.

The author is being a little too clever for his own clogs here. "The helm atop his ebon armour"?! Come on. :) Anyway... the "last boss" is found in video games, ie. the most powerful opponent at the very end of the games. I guess "class" means the classification (type of opponent). I really don't know if the writer actually translated a comment from Japanese, or just made up the phrase. My money's on the latter. I think he was trying to make a description which sounded like a bad literal (nonsensical) translation, or at least something like Japlish.

It's in an article about a kind of Japanese warrior's helmets called "kawari kabuto," but "lastboss" seems to be a game word, as others said. Thanks, Michael!

Is this sentence from a game of some sort ?