Hey, guys! Today, I'd like to share my ideas on the fabled "Lunar Update," as well as the "Lunar Boss."

We all know the Lunar Update is supposed to end Terraria and begin Terraria 2. This means it's gonna be HUGE, since the developers would probably want to keep us occupied, while T2 is still in development. Could the Lunar Boss, which I am codenaming Luna, be Cthulhu, an idea believed by some to be scrapped? We all know since Luna will be the final boss (probably), it will be extremely powerful.We're talking being able to summon other bosses.
The moon has always played a valuable role in Terraria. The phases affect certain things, such as exclusive items from Vendor NPCs, the Halloween-themed Pumpkin Moon, and the dreaded Blood Moon (also Frost Moon).
Let's think. The Blood Moon could be Luna's rage, and Luna could be telepathically linked with all monsters, including innocent Bunnies, Goldfish, and Birds. This rage may telepathically rub off on monsters, causing them to corrupt and become much more aggressive.
Since the Pumpking is a Pumpkin Moon exclusive boss, it could be a form of Luna's rage, or one of it's minions. The Mourning Wood (also a Pumpkin Moon boss) could be a hyper-corrupted tree, caused by Luna.
Luna seems to have power over everything evil in Terraria, and even some innocent creatures, as I mentioned before. This brings us to my next phase of the theory, the Frost Moon.
Christmas, as we know, is the purest and holiest time of year. Everything, and everyone is happy. This makes Luna angry, as happiness spreads love, not evil. This is why it created the Frost Moon, to make Christmas look evil, and ruin Christmas and Santa Claus's good name, causing people to lose faith in them, and hate Christmas. Santa put a stop to this, by sealing all this hate and negativity in a box, the Naughty Present. If it were to be opened, all evil would be released. This would awaken the Frost Legion, and corrupt Christmas elements, such as a Christmas Tree becoming Everscream, Santa's sleigh becoming Santa-NK1, and much more, such as zombified elves. The Ice Queen is known to be the equivalent of Pumpking for a Frost Moon, possibly making it another form of Luna's rage, and/or a minion of it.
It seems Luna is using it's power over the moon to it's advantage, by using it to spread hate and evil, but why does it want to do this? Was it born like that? Is it cursed like the Old Man was cursed by Skeletron? Could there be a Solar Boss that opposes it, which is a spreader of goodness? We will have to wait patiently for the Lunar Update to find out...
I think Luna was a kind, adventurous woman, and one day, she came in contact with a powerful item, like the Moon Charm, but much more powerful, that gave her unimaginable powers, and also made her evil to the core. She ravages Terraria with monsters, created with her Lunar Power. Everyone was too scared to challenge her. Then, one day, a brave warrior (your Guide in present day) challenged Luna. After an epic battle that nearly ripped apart the fabric of time and space, the Guide banished Luna to the moon for thousands of years (a little MLP reference in there).
It is believed that Luna will be signaled by an invasion of some sort, similar to Goblin Army, Pirate Army, and Frost Legion. If this is true, could Goblins, Pirates, and Snowmen be some form of extraterrestrials invading the planet? And why does Luna spread evil and terror? It already has an army of aliens, ready to invade. It could be to distract heroes capable of defeating it, while they invade the planet.
Although The Underworld is basically the Earth distance-wise opposite of the moon, I believe it to have a strong connection with Luna. Demons and stuff like that is expected in Hell, especially Bible-wise, but a giant wall of human flesh and mixed up body parts? Not expected. Since the Guide must be killed to summon Wall of Flesh, it is very probable that since the Guide is such a powerful being, his spirit mixed with the anger and hate of Luna and its minions created WoF. And when it is defeated, the hate and anger dissipated, explaining the X and/or V-shaped Corruption formation, which begins to spread.
The Hallow, i believe to be the love and hate of Terraria combined, and Sol (Solar Boss's codename) is the only pure source of love an happiness, since the Hallow contains evil creatures, even Unicorns attacking you.
But the origin of Sol? I believe Sol is some sort of Sun god, and Luna is a Moon goddess, and all this may follow the Chinese legends of Yin and Yang. Yin represents passive, cold, death, Winter, Female, night, even, Moon, and water, while Yang represents active, hot, life, Summer, Male, day, odd, Sun, and fire. These 2 are believed to be a duality, meaning one cannot exist without the other. This could mean the final battle will be so epic, Terraria, Luna, Sol, and everything will be destroyed, but will be reborn in T2.
Adding to Luna's story, we know the Guide banished Luna to the moon, but not alone. He had Sol's help. Sol was a timid young man, same age as Luna. He then met an old man, who said he was hungry. Sol was shy, but not mean. He gave the old man an apple. In return, the old man gave Sol a necklace with a sun charm on it. Sol never saw this old man again. When he put it on, he was filled with courage and righteousness. But this was at the same time Luna found the moon necklace and put it on. While Sol spreaded joy and happiness and love, Luna destroyed everything in sight. Then, Sol one day saw the Guide (called the Warrior back then) running and throwing knives at something. Sol asked what was going on, and the Warrior explained how Luna was destroying Terraria. Then, the two heroes agreed to fight Luna together. They got everyone they could find, and told them to think positive. These positive thoughts gave Sol and the Warrior power, while weakening Luna and her army. They then crafted a legendary weapon, and filled it with the positive energy. They then warned Luna to stop and leave the land she had ravaged, or die. She was bonded with the Yin Amulet. She could not agree to leave, even if she hadn't lost her mind. Then, the battle started. After what seemed like days of intense battle, they struck the mighty weapon into Luna, when the Sun and Moon were in perfect balance in the sky, and she evaporated into the moon. As this happened, Sol began to disappear. The Guide was in tears. But he did not have time to mourn for long. The land was drenched in blood, and thousands of buildings were destroyed. The Warrior and the people had work to do. He then destroyed/hid the legendary weapon, as he knew if it fell into the wrong hands, it would be catastrophic. But this weapon will be wielded again by a hero pure of heart (the player, hint, hint), when Luna and Sol return to battle once more...
Now, I will discuss the topic of how this may relate to Cthulhu, which I mentioned earlier. In HP Lovecraft's stories, it says the stars will aid in Cthulhu's escape. The Sun is a star. The Sun and Moon both affect the tides. R'lyeh, the corpse of a city where "dead Cthulhu waits dreaming" is underwater, which is affected by the tide. See the connection? It is also said Cthulhu's daughter, Cthylla, has the ability to give birth to Cthulhu if he somehow dies. In a story, Cthylla gives birth to Cthulhu in the distant future, where Cthulhu is destroyed. For all we know, Terraria could be post-apocalyptic, or even takes place in an alternate dimension, where the timeline is completely different. All this and more will be revealed in the Lunar Update, and/or Terraria 2.
Now for how to start this battle for the fate of Terraria. after the invasion is defeated, a refugee of the aliens (sorta like the Goblin Tinkerer) will tell you all this, and will tell you to go find Sol and ask him to assist you in battle with his Yang Power, you must go to the temples of Heaven, Wind, Water, Mountain, Earth, Thunder, Fire, and Lake (which will generate after the alien invasion is defeated), and retrieve the 8 Legendary Objects, and he will give you the Tao Forge, which you can use to craft the 8 Legendary Objects into an item that has affects similar to Obsidian Skin Potion and Obsidian Skull, except more powerful, and you will be able to build a Rocket Ship to travel to the Sun, and talk to Sol. When you reach the Sun, the Alien Refugee will reveal he was a spy all along, and try to take your Tao Charm so he can attack Sol, and you must throw him on the eternally burning fire of the Sun, and he will die painfully right in front of you. Sol detects this disturbance, and believes you are an intruder. He will at first try to attack you, but realize you are a good person. He will then agree to help you, and you must use the Rocket Ship to go to the Moon, and begin the final battle...
BUT! The Alien Spy will stowaway on your ship, apparently being immune to the Sun's fire, and faked his death, and will once again try to send you into the eternally dark vacuum of space, but once again be defeated, this time for good, since he was too foolish to not expect the Tao Charm to allow you to survive in space without air, much like a psychic astronaut suit. THEN, you shall reach the Moon, and begin the battle, after getting past Sol's army.

How did you guys like that one? Please leave a comment, and tell me what you think, and by the way, thanks for a warm welcome to the wiki, I appreciate it!