Nazgul

Nazguls' are a difficult-to-defeat enemy, similar to the Dungeon Guardian with the exception of damage - that is, not causing instant death, but still dealing a large amount of attack damage.

Spawning
The Nazgul has a very small chance of spawning in the night, and has no chance of spawning in the day, making them arguably the hardest mob to defeat if the player is unprepared.

Trivia

 * The Nazgul is one monster that spawns when fighting Sauron*, a possible reference to The Lord of The Ring movie.
 * Nazguls have 2 attacks, Melee and a spell. The melee does 130 damage, and the spell does 30 ( Needs to be confirmed , Tested once)

Tactics
While using low-level spells and weapons, the Guardian Ultima Weapon * Barrow Blade is the weapon that dispels all of their armor, making them easier to defeat. Or if the player doesn't have it, running off of the screen to cause it to despawn is another option.