Viking Helmet

It has no set attached, so putting it on means giving up the bonus from whatever armor you are already in. By the time you can farm it that trade is almost never worth taking. Where it does fit is early, when your chest and legs are mismatched leftovers anyway and raw defense is the only thing you are counting.

There is also the obvious use: it looks good, and a helmet with no set obligations is easy to slot into a vanity outfit.

Once the world converts, Armored Vikings take over most of the ice cave spawn pool and the undead version gets noticeably harder to find. Grab it while the ice biome is still simple. If you already missed that window, an Undead Viking Statue wired to a timer will grind out the drop in a corner of your base while you do something else.