Ice Queen

The Ice Queen is the final boss during the Frost Moon. She is the equivalent to the Pumpking from the Pumpkin Moon, being the Frost Moon's most powerful monster, coming in at the last waves of the event, and being the event's only flying boss. As of the 1.2.4 update she appears to be the third strongest enemy type in Terraria (the strongest being the Dungeon Guardian and the second being Duke Fishron).

Her behavior is characterized by astonishingly fast movement and evasive maneuvers while making constant attacks against the player. There are two ways to counter this - either use wings and possibly movement enhancing accessories to chase her, or a Cross Necklace/Star Veil combined with healing (Spectre armor/Vampire Knives) to mitigate her damage. If not using the Knives, a powerful ranged/magic weapon with high fire velocity is recommended. If a bullet-firing weapon is used, it may be a good idea to equip the Rifle Scope (or, even better, the Sniper Scope) as she will be off-screen for a majority of the battle, and is sometimes difficult to keep track of during the fight.

Trivia

 * A possible Adventure Time reference, though unlikely considering the prevalence of ice and snow queens in popular culture. Some examples include the titular character of The Snow Queen (Hans Christian Andersen, 1845) and the White Witch from The Lion, the Witch, and the Wardrobe (C. S. Lewis, 1950), as well as being a common insulting term meaning a compassionless, callous, aloof woman (particularly one who responds to suitors' expressions of love with disdain).
 * This is more likely a reference to Snow Princess Kaguya from the Sailor Moon S movie (and manga short story), as the general character design is exactly the same, including the headpiece and placement of the skirt.