Eye of Cthulhu

The Eye of Cthulhu is one of the pre-Hardmode bosses in Terraria. It is usually the first boss that new players encounter, and is thought to be the easiest boss by the game developers.

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Summoning
The Eye of Cthulhu (often abbreviated as EoC) can be summoned by using a Suspicious Looking Eye at night. Alternatively, it can randomly summon itself with a 33% chance each night if the following criteria are met: If these conditions are met, the player may get the message "You feel an evil presence watching you...", which means that the Eye will awaken soon. After a few minutes the message "The Eye of Cthulhu has awoken!" will appear and the boss will promptly start to attack the player. However, if "You feel an evil presence watching you..." and you aren't ready to fight the Eye of Cthulhu, you can opt out of fighting it by simply exiting and restarting the world, or alternatively restarting the game.
 * At least one player in the game world has at least 200+ health.
 * There are at least four NPCs in the world. Note that the Old Man counts as one of these.
 * The Eye of Cthulhu has not already been defeated in the world.

Strategy
The Eye of Cthulhu has two phases that it goes through as it does battle with the player. The first phase, a giant eye, has 15 attack power and 12 defense, while the secondphase, a giant eye with a gaping jaw in place of its lens, has a 23 attack power; however, it has no defense. A good weapon to use during the first phase is the Starfury : It being a broadsword means you are able to fend off The Servants Of Cthulhu and use it's ranged ability to fight Cthulhu. Keep in mind that the EoC is immune to spikes so don't make an arena like this--

(I will put in picture while im on my computer)

Preparations
As the Eye of Cthulhu can only summon itself if your max HP is 200 or more, you can delay its arrival by holding onto any Life Crystals that will increase your max HP to 200 or more until you feel that you're ready. That way, you'll be able to wait until you can craft Gold Armor and a Gold Bow. However, if you're not patient enough to want to grind for gold-tier equipment, then you should at least have Iron Armor or better, an Iron Broadsword or better, and, most importantly, some sort of ranged weapon, such as Shuriken, a Throwing Knife, or a bow (preferably Iron or stronger) with Flaming Arrows. Also, bring along any Lesser Healing Potions in case you need to heal. If possible, get Glowing Mushrooms for Healing Potions instead.

If you are feeling like just rushing in the game, and are skillful or lucky enough, you can make a Topaz Staff, get some Fallen Star for extra mana, and get some Iron Armor. With 160 or 200 health, and Healing Potions, you can beat the eye with just dodging skills and a nice aim.

Another strategy would require the player to find an explosive weapon underground as soon as possible and have the Demolitionist spawn. Once the player had access to a steady supply of Bombs or Dynamite, and either a Featherfall Potion or a Hook they would then have to go to the Corruption and destroy a Shadow Orb as soon as possible in order to gain a Musket. Once the Arms Dealer arrived the player could then work out a way to obtain 5 gold and 35 silver for a Flintlock Pistol and 500 Musket Balls. One could use as low as copper armor for the Eye of Cthulhu provided they made sure to heal themselves properly before moving to phase two. As the EoC could almost be described as passive during phase one it would allow the player enough time to heal before moving on.

The following consumables may also be of some help:
 * Ironskin Potion: This will increase your defense by 8 for five minutes, allowing you to take up to four less damage from the Eye of Cthulhu's attacks.
 * Thorns Potion: For two minutes, enemies will take half of the damage that you take. This can make the battle go a bit quicker, though you'll probably want to try to avoid the Eye's attacks instead, especially during phase two.
 * Archery Potion: For four minutes, the damage dealt by your bow & arrow increases by 20%, which will help decrease the time of the battle greatly.
 * Regeneration Potion: For two minutes, your health regeneration is augmented, allowing you to recover more lost health in a faster period of time.
 * If you have explored enough, and you are better with melee weapons in general, a Gravitation Potion with a silver or golden Broadsword may also be of use.

Weapons Analysis

 * Shurikens: These weapons can be thrown rapidly and can penetrate through enemies, meaning that these can not only kill Servants of Cthulhu in one hit (thanks to their attack power of 10), but they'll also go on to damage the Eye itself. Although they only deal 4 damage to the Eye's 1st phase, due to their piercing mechanics, they'll often damage the Eye more than once, making them an ideal weapon. They're pretty easy to obtain as well, since not only are they commonly found in pots, but they can be purchased from the Merchant at any time for only 20 copper each. Throwing them straight in the iris of the Eye will hurt the servants as well.
 * Throwing Knives: Although they are a bit harder to get than Shurikens (they're also common in pots, though the Merchant will also sell them during a Blood Moon for 50 copper each), they deal a bit more damage, and can likewise be thrown rapidly and penetrate enemies. They deal 6 damage to the Eye of Cthulhu's 1st phase, and can hit the Eye more than once per throw. However, if you're skilled (or lucky) enough to have access to Vile Powder (which is crafted with Vile Mushrooms from the Corruption), you can use that to upgrade your Throwing Knives into Poisoned Knives, which deal 7 damage to the Eye's 1st phase, and can even poison it, inflicting even more damage in the process.
 * Bow & Arrow: Although you cannot fire a bow and arrow as rapidly as you can throw Shurikens or Throwing Knives, it's on here due to its potentially high damage output. A Golden Bow with Flaming Arrows deals 17 damage, which means that it'll deal 11 damage to the Eye's 1st phase. While Flaming Arrows cannot penetrate enemies like Shuriken or Throwing Knives can, it does have a chance of setting enemies on fire, damaging the Eye even further. On the other hand, a Golden Bow with Unholy Arrows (which are made out of normal arrows and Worm Teeth, which are dropped from Devourers) deals 19 damage (13 damage to the Eye's 1st phase), and can penetrate enemies like Shuriken and Throwing Knives can, though it'll travel too fast to damage the Eye multiple times.
 * Musket: If you have Silver Bullets, this could be of use. Regular Musket Balls work as well, but you should have Golden or Silver Armor and Lesser Healing Potions or Healing Potions, as its very slow speed can make it difficult to do consistent damage. With Musket Balls it does 30 damage in the second phase, and 18 in the first.
 * Rare Weapons: Most rare weapons you find in chests in the game will help you a lot. For example you can use an Enchanted Boomerang with your armor (Gold or better) and Healing Potions you should be able to defeat the eye easily.

Arena
Mobility in all directions is important in avoiding the Eye of Cthulhu's attacks, and chances are that you won't have access to such items as Rocket Boots or Hermes Boots. Due to the player's limited jumping skills, it is strongly advised that you build a boss arena that consists of multiple rows of wooden platforms. Finally, the arena should have some tall walls to keep other enemies from entering the arena, though a ceiling isn't required. With no ceiling, Shurikens, Throwing Knives, and Arrows that are thrown upwards would fall back into the arena, and can still damage the Eye of Cthulhu during the descent. Keep in mind that the EoC is immune to Spikes--

Try to have an open roofed arena because a falling star can do 1000 damage to the eye of cthulhu!

Also remember to light the arena with torches, as the battle can only take place at nighttime.

The battle
The Eye of Cthulhu can only be summoned during night-time and will disappear without dropping any loot if the player does not kill it before daytime. Therefore, the best time to summon this monster would be soon after nightfall. That way, you'll have pretty much all night to defeat it.

Phase 1
The Eye of Cthulhu will try to hover high above the player and shoot out a Servant of Cthulhu every couple of seconds from the iris. The Servants of Cthulhu each have an attack of 12, but only 8 HP, which is low enough for them to die from one Shuriken. While the Eye is summoning Servants, you should be aiming for the pupil, as that's where the Servants will spawn from. If any stray Servants do get too close, then a Broadsword is useful for getting them out of your hair.

After four to six Servants of Cthulhu have been launched, the Eye of Cthulhu will start to dive towards the player three times. It has an attack power of 15, which can be damaging if care is not taken. Still, players should try to avoid these dive attacks whenever possible, changing to a higher or lower level if need be. Note that it's possible to lead the Eye's dive attacks by jumping before it attacks.

Phase 2
Once the Eye of Cthulhu reaches 1400 HP or below, it'll start to spin around violently, shedding its iris "covering" (and lowering its defense to 0) to reveal a jaw full of sharp teeth, and setting the stage for phase 2 of the battle. While this happens, it is recommended to damage it as much as you can, due to lack of movement and retaliation. While the Eye no longer spawns Servants of Cthulhu during this phase, it hovers closer to the player to ensure that its dive attacks (which now do 23 damage) are more accurate. It also attacks quicker, making it more difficult to lead its attacks.

However, while the Eye of Cthulhu's speed is usually faster than the player, its acceleration and deceleration is rather poor. A clever player can exploit this by running in one direction, causing the Eye to give chase, then immediately stopping and running in the other direction, causing the Eye to drift away as it struggles to keep up with the player.

Trivia

 * The Eye of Cthulhu is named after the fictional extraterrestrial entity, Cthulhu, created by horror author H. P. Lovecraft.
 * The correct pronunciation of Cthulhu has long been debated. KAH-THOO-LOO and CHA-THOO-LAH are two commonly argued pronunciations. In reality, Lovecraft intended the name to be unpronounceable, as the creature predated the English language.
 * In patch 1.0.3, during multiplayer, if you pulled out a Suspicious Looking Eye, an Eye of Cthulhu would spawn and your Suspicious Looking Eye would not be taken away.
 * A boss resembling the Eye of Cthulhu, known as Monoculus, was introduced to the FPS game Team Fortress 2, around Halloween, 2011. Monoculus appeared as a large eyeball shooting smaller eyeball projectiles in a similar manner to the Eye of Cthulhu.
 * The Merchant has claimed he occasionally feels an evil presence watching him, perhaps stating that not unlike Skeletron, his curse monster is the Eye of Cthulhu. He also states that he hopes a 'scrawny little kid' like the player isn't the only thing standing between the NPCs and the Eye of Cthulhu. The Old Man says insults quite simular to this also.