Deerclops

The Deerclops is a pre-Hardmode boss. It is one of the final bosses a player may encounter during pre-Hardmode. It can be summoned manually using a Deer Thing at any time of day in the snow biome.

Spawn
The Deerclops can be summoned using a Deer Thing at any time of day as long as the player is in the snow biome. Although it can be summoned immediately, the Deer Thing requires either Demonite or Crimtane ore to craft, which isn't readily available until after the Eye of Cthulhu has been defeated.

It also spawns naturally at midnight during a Blizzard.

Battle
The Deerclops has three attacks:
 * Summoning a wave of ice spikes from the ground
 * Throwing chunks of ice into the air
 * Summoning damaging nightmare hands that chase the player.

The Deerclops its self is invincible unless the player is within a certain radius of it.

When far from the player, the Deerclops can pass through blocks, but otherwise remains on level ground.

Trivia

 * The Deerclops is the only boss in Terraria to not de-spawn after killing the player.
 * The Deerclops is one of 2 boss fights that consists of a single destructible entity, the other being the Empress of Light.
 * All other boss fights have either multiple bosses, multiple segments or boss minions that play a part in their boss fights.