Ancient Frost Spirit

The Ancient Frost Spirit is a hardmode boss that can be fought in the Tundra/Snow biome after all 3 mechanical bosses have been defeated. The boss turns all Corrupt, Crimtane, and Hallowed blocks into Eternal Frost biome blocks when defeated. While the Ancient Frost Spirit is alive, Boss 2 will play.

Summoning
"The Ancient Frost Wisp has Awoken!"

After all 3 mechanical bosses have been defeated, all Snow biome enemies have a 5% chance to drop the Frozen Ectoplasm item, which when used in the Snow biome summons the boss.

Attacks
Phase One: During the First Phase, the spirit will be in it's wisp form. It will fly above you similar to Queen Bee, Ocram, and Eye of Cthulhu while shooting icy bombs, which explode upon contact with any surface and move at a speed similar to Plantera's Thorn balls. The boss will also shoots small ice shards similar to the crystal storm that home in towards the player and disappear after 2 seconds.

Phase Two: Once reaching half health, the spirit will disappear into the ground and transform into a humanoid spirit with an Ice Sickle. It will rapidly shoot Ice sickle-shaped projectiles that shoot forward infinitely through blocks. After 2 seconds of this, it will shoot a barrage of ice shards along with occasional icy bombs for another 4 seconds, then repeat.

Phase Three: Once it reaches a quarter of its max health, it will transform into a smaller version of it's first form the size of the Wisp light pet. It will constantly flight around at high speeds and shoot icy shards until it dies. If the player touches the boss' body in this phase, they will be given the "Frozen" debuff.

Enraged Behavior: Whenever the boss is outside of the surface snow biome, it gains double health and does double damage. Returning to the surface snow biome removes these buffs.

Stats
First Form:

Ancient Frost Wisp 1 AI

30/ 60 Melee damage, 60/ 120 icy bomb damage, 40/ 80 Icy shard damage

33,000/ 46,000 Max Life

7 Defense

100% Knockback Resistance

Second Form

Ancient Frost Wisp 2 AI

45/ 90 Melee Damage, 75/ 150 Icy bomb damage, 55/ 110 Icy Shard damage, 70/ 140 Ice Sickle Damage

10 Defense

100% Knockback Resistance

Third Form

Ancient Frost Wisp 3 AI

10/ 20 Melee Damage, 60/ 120 Icy Shard Damage

0 Defense

100% Knockback Resistance

Drops
15-40 Ectoplasm (35-50 in Expert)

15 Gold Coins (18 in Expert)

One of the following will always be dropped, but only one

Eternal Ice Sickle (33%)

Eternal Icy Storm (33%)

Eternal Icy Bomb Launcher (33%)

Treasure Bag:

Eternal Wisp (100%)

Extra Info

 * The player will get the Achievement: "Let it go" when defeating the boss.
 * The boss can be fought prior to hardmode, but will not spread the Eternal Frost unless it is defeated in Hardmode
 * Defeating it before Plantera would give mages a way to get Spectre Armor before the Plantera fight