The Watcher

The Watcher is the first boss you will usually encounter, since it is very easy to summon and it also tries to summon itself if not already defeated in a world. It can only summon itself if there has already passed 5 in-game days and the player has killed 30 Little Eyes. It resembles a robotic and strong, black steel body that has a few buttons on it. However, it has an eyeball inside a glass bowl instead of a head, which it's name comes from - The Watcher.

Fighting The Watcher
Fighting The Watcher should be easy even for new player, as long as they have decent gear. It tries to perform very simple attacks at you, which should be easy to avoid.

Getting Ready
You should prepare well before fighting The Watcher if it's the first boss you fight. You should have at least a Silver Sword or some sort of a Spear, a Copper Bow or a Tin Bow with Sharp Arrows, a full set of Copper Armor or Tin Armor, or if possible, Bronze Armor. Also bring some Poor Healing Potions, or if you have Decent Healing Potions, use them instead, since healing 40 at once instead of 25 is a lot better. You could also have some sorts of other potions, like a Shield Potion or a Regeneration Potion. You should also make a bridge like arena with multiple levels, to dodge The Watcher's punches easily.

Summoning The Watcher
Summoning The Watcher is relatively easy. First you need to craft an Eye Plate, which is created with 10 Iron Ingots, 5 Copper Ingots, 5 Tin Ingots and a Small Eye, which is dropped from Little Eyes at a Tinkering Table. The Eye Plate can only be used at night in order to summon The Watcher. If you use it at daytime it will only spew out some sparks and hurt you by 25 health. It is recommended to spawn it in an arena. It will soon appear dropping down from the sky. Beware through, since it will hurt you with 50 health if it drops right on you when he arrives.

Fighting The Watcher
When The Watcher tries to punch you, jump backwards in time, or it will succesfully hit you and can deal moderate damage. If you out of it's reach, it will randomly eather walk up to you and hit you, or it will jump up and try to flatten you. If it jumps it will go through any blocks on top of it, so any kind of trapping method wont work. It is rather easy to dodge it when it tries to get to you, so you could just keep running away from it and keep shooting arrows at it. It only has 1,300 health, so it is rather easy to kill it with a really destructive melee weapon or with an effective ranged weapon. Just by dodging its attacks and shooting or hitting it continously should kill it in a matter of time.

Aftermath
After you have killed The Watcher in the world, it will not spawn itself again. However you can summon it as many times as you want afterwards, as long as you have the Eye Plate to summon it. It drops Steel Ingots, which can be hard to get early in game and are pretty useful, Mechanical Snaps which are used in advanced machines and can't be obtained from a lot of sources. It also drops 1 Watching Eye, which can be used in many ways: It can be placed on the helmet slot and you will have a huge eye instead of your head, it can be used in crafting some advanced items or it can just work as a decoration, since you can place it in a jar.

Trivia

 * The Watcher is like all the other eyes (Little Eyes), but has a lot larger body, and has claimed itself the leader of the eyes.
 * The Watcher's body weighs 1,350 kilos.