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, Zombie Merman, and the two other Hardmode-exclusives, the Blood Eel and Hemogoblin Shark. The Dreadnautilus is rarer than the other four Blood Moon fishing enemies, spawning from only 1 in 10 spawn attempts on average. It is widely considered the most difficult of these five enemies.

Attacks

 * Spins around the player for roughly 3 seconds. It will damage the player upon contact.
 * While this attack is charging, Dreadnautilus will face away from the player and will reflect projectiles.
 * Fires a rapid spray of red projectiles at the player. During this attack, the Dreadnautilus will turn invincible.
 * Briefly glows crimson and summons three red symbols. After a moment, each symbol will summon a Blood Squid, which will attack the player with a ranged projectile.

Tips

 * Summon weapons are useful for handling the Blood Squids while the player focuses on the Dreadnautilus.
 * 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 you to skip past other blood moon fishing enemies and unrelated catches, letting you fight the Dreadnautilus more over the course of the night.
 * Of the 3 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. When you see the enemy charge up for the spin attack, temporarily leave the box and re-enter after the enemy is finished or you can tank it by using Campfires, Heart Lanterns and Honey. (and Regen Potions if you are 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.

Trivia

 * Similar to the Sand Elemental and the Ice Golem, the Dreadnautilus is not a boss or mini-boss even though it has stats comparable to a boss. This is evidenced by its lack of a health bar and the fact that it drops a banner, not a trophy.
 * Despite this, it is referred to as a miniboss in the 1.4 changelog.
 * 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" in an official trailer.
 * The dev team may have gotten the idea for adding shellfish boss to the game from an April fool's tweet by Cenx, stating that a boss called the Conch of Fish would be added, playing off the the acronym WoF, for the Wall of Flesh.