Armored Viking

It appears in the ice biome once a world goes Hardmode, with no summon and no event behind it. Same silhouette as the Undead Viking with considerably more behind it, so a snow cave that felt manageable an hour of play ago is not any more.

The extra defense is the part that changes how you fight it. Weapons that win by hitting often lose a large slice off every individual hit, so fast early Hardmode gear stalls badly here while a single heavy blow keeps most of its value. Bring the slower weapon and take the trade.

It used to let itself into houses. These days it only works doors and tall gates during a Blood Moon, or occasionally in a graveyard, so a base in the snow is much less exposed than it once was. The Ice Sickle is the reason to go looking for one, among the better melee options open right after the wall falls, and the ice caverns are its only source.