Dreadnautilus

The Dreadnautilus is a Hardmode enemy summoned by fishing during a Blood Moon. It is one of five enemies that can be summoned by fishing during a Blood Moon in Hardmode, the others being the Wandering Eye Fish and Zombie Merman (which are also available in pre-Hardmode), and the Blood Eel and Hemogoblin Shark. The Dreadnautilus is rarer than the other four Blood Moon fishing enemies, spawning from only of spawn attempts, on average. It is widely considered the most difficult of these five enemies. If the Dreadnautilus is not defeated by dawn (4:30AM), it will fly away and despawn.

Attacks
The Dreadnautilus will cycle through attacks in the following sequence:
 * Face away from the player and charge up for about 1 second.
 * It can reflect most projectiles during this charging phase on all difficulties. For the complete list, see list of reflected projectiles.
 * Dash directly towards the player. After reaching the player, it will spin around the player. This attack last up to about 3 seconds, and it will stop dashing/spinning after 3 seconds has passed.
 * Face to player and charge up for about 1 second. It will not reflect projectiles during this phase.
 * Fires three rapid sprays of red projectiles at the player and recoils after each burst.
 * Repeat dash and projectile attack twice.
 * Briefly glows crimson and summons three red symbols. After 1.5 seconds, a Blood Squid will emerge from each symbol, and the symbols will despawn again after another 1.5 seconds. The Dreadnautilus will only perform this attack if the player is less than 125 tiles away. Symbols can only be summoned in the air, water, or honey, with an unobstructed line of sight to the player, and when there are fewer than three Blood Squids currently alive.

Tips

 * The Sonar Potion will detect if the Dreadnautilus is about to be hooked, and can give the player a moment of warning to back away and activate their potions.
 * Using the Chum Caster gives the player a greater chance of finding the Dreadnautilus and other Blood Moon fishing enemies. Using Sonar Potions will allow the player to skip past other Blood Moon fishing enemies and unrelated catches, letting them fight the Dreadnautilus more over the course of the night.
 * Summon weapons are useful for handling the Blood Squids while the player focuses on the Dreadnautilus. The Blade Staff is a great choice due to its defense-ignoring capabilities, and will ignore all of the Dreadnautilus' defense.
 * Inflicting the Ichor debuff, e.g. via the Golden Shower, can be helpful for lowering its high defense.
 * Hiding under blocks is useful as it blocks Blood Squid's projectiles, as well as the red spit projectiles the Dreadnautilus employs.
 * Of the three attacks that the Dreadnautilus employs, only the spin attack will phase through walls, since the enemy itself also phases through walls. A good strategy to cheese the enemy is to build a box and use weapons that can attack through the walls like the Daedalus Stormbow with Ichor Arrows, or use Summons. The spin attack only hits once per cycle, since the AI defaults to spinning around the player without touching them if they stand still, so the player can tank it by using Campfires, Heart Lanterns and honey (and Regeneration Potions and Well Fed buffs if the player is in higher difficulties).
 * Since this fight takes place at night with a possibility of rain, visibility can be an issue and can make projectiles difficult to perceive as the blood-colored rain is too similar in color. Using a Hunter Potion, along with a Night Owl Potion and Shine Potion, will help with visibility.
 * If Blood and Gore is turned off in Settings, the red spit projectiles will be covered and followed by thick white cloud, and it can be harder to distinguish projectiles among the clouds. However, the white cloud can make projectiles more visible during night.

Trivia

 * Despite being officially considered a mini-boss, Dreadnautilus has a lot of similarities to a boss. It is considerably harder to defeat than regular mini-bosses, has complex patterns reminiscent of a boss including spawning supporting enemies, and even its own achievement.
 * Leinfors goes over this in the official Terraria Discord, expressing that Re-Logic had problems deciding whether Dreadnautilus should be an event boss or just a normal mini-boss, with a trophy even being made for it.
 * It resembles a, the creature after which it is named.
 * Its name is a portmanteau of ', a type of early 20th-century battleship, and ', a marine mollusk of the Nautilidae family.
 * Its original name was Blood Nautilus, as seen in the official Journey's End trailer.
 * Re-Logic may have gotten the idea for adding a shellfish boss to the game from an ' forum post by Cenx, stating that a boss called the Conch of Fish would be added, playing off the acronym WoF for the Wall of Flesh.
 * The name of the achievement unlocked by defeating it is a reference to the song Don't Tread On Me by Metallica.