Disclaimer: This is not part of a mod nor part of Terraria.
Terraria's twin planet, Obscurus, was first discovered several years before the first Terrarian hero came to be. It was found by one of the Goblin Empire's astronomers, and the news was reported to the Goblin King. Nobody took much notice of it, as it didn't seem to have much effect on Terraria. It was noted for it's hazy and dark appearance, and it wasn't spotted sooner due to the fact that it nearly couldn't be seen during the day nor night.
However, a few years later, after the Terrarian hero had risen to great power after defeating Skeletron Prime, it was noted that the planet seemed to behave oddly. The gasses on it's surface seemed to be spinning faster than ever, and colossal hurricanes appeared to be covering its surface, causing alarm to the astronomers at the Goblin Empire. Assuming that the problem was due to the Terrarian hero's work, the Goblin King dispatched legions of trained soldiers to destroy his house. However, their efforts were in vain and their soilders were defeated as fast as they could send them. Finally, calling off the attack, the Goblin Empire closed their gates, placed the entire area under high alert, and hoped for the best.
The Beast arrived soon afterwards.
Five days killing Skeletron Prime the first time, each day has a 80% chance of a Goblin Army attacking. This will not be an ordinary Army, though, as it will contain several new enemies. Each army will attack relentlessly, only pausing each night. The event will only stop once the player has slain two thousand enemies belonging to the army.
Several NPCs will gain several new quotes during this event, such as:
"I don't like this. The Goblin Empire doesn't usually send those generals into battle."
"Something on Obscurus seems off. What, you've never heard of Obscurus?"
"I've heard rumours that the Goblin Empire is desperate to kill you."
"Obscurus, you say? Hmm, I can't say I've ever heard of that place." (Only after speaking to the Goblin Tinkerer)
"I've heard about a rumour of this planet... Oh, right, you're here to buy things."
"[Goblin Tinkerer's name] has been acting wierd lately. Have you spoken to him?"
"Look what them Goblns brought to town!"
"This ominous feeling... it feels stronger and more evil than the corruption. You've got to stop it, somehow."
"Obscurus? I've heard it from somewhere. Can't remember who or what that is, though."
Runs extremely fast and deals 60 damage at a fast rate, but has 60 health. Can fit through one-block spaces by "crawling", and can climb up walls. Will attempt to enter the house via any opened doors or areas, but cannot break down doors.
Normally spawns accompanied by fifteen other random Goblin Army enemies, including new ones. Has a thousand health and may respawn dead enemies three seconds after they die. When killed, all enemies on screen die too.
The Beast lived on the planet of Obscurus before it was propelled to Terraria by a exceptionally strong hurricane. It was unlike most of it's type whom died flying through the vast distances to get there, it was several times larger, and much stronger. It was extremely agile, like the rest of its kind who could appear to traverse lengthy distances in the blink of an eye, and it's huge, retractable claws could tear through nearly anything.
Within two to three days after the Goblin Army retreats, The Beast will awake at night.
About the size of Skeletron, this land-based enemy follows Unicorn AI. It will charge at the player, jumping over any holes in it's path, and attempt to hit the player. It cannot be damaged from above by any ranged items, including magic and boomerangs. Upon contacting a wall it can't jump over (It has a jump height about that of the King Slime) or a hole in which the player is hiding, it will use it's claws to smash through them, creating an explosion which destroys anything in about ten blocks' radius. It moves twice as fast as a player with Hermes Boots on at maximum speed.
It deals 80 damage and has 30,000 health, with a defence of 40. However, this defence will drop down to zero during the day. The explosion it creates when smashing through blocks deals 100 damage to the player. It does not despawn, and will continue chasing the character, smashing through walls to get to him. Leaving the game will cause it to appear with full health ten seconds after the player rejoins the game. It will only go away if a boss-summoning item is used, but will be back during the next Blood Moon.
When defeated, it drops ~1 Platinum coin, and will drop ~twenty Shadows.
Obscurus was created alongside Terraria, thousands of years ago. Before then, Terraria was clean of all traces of Corruption, while Obscurus didn't contain any Hallow. The two planets orbited each other when one day, a meteorite slammed into Obscurus, sending some of the planet's mist over to Terraria. This, in time, created the Corruption.
The Corruption on Terraria was fueled by the Eater family who fed on souls to keep the Corruption alive. However, due to the nature of Obscurus, the Corruption there thrived on the planet's Shadows. These creatures served to taint any creature who consumed it, but provided a plentiful source of food for the planet.
After the Beast is slain, a new biome will be created randomly on the surface of the map with the message "A corrupted meteor has landed!" This area is ~700 blocks wide but only covers the overground layer, and will not spread. It spawns over the COrruption, and if there is no source of corruption for it to spawn over, it will spawn in a random location in the map.
This new biome, The Shadows, is much like the previous Corruption but with the difficulty ramped up to a whole new level. Corrupted grass will be replaced by Shadow grass, which is a darker hue than the Corrupted Grass. Ebonstone will be replaced by Shadowstone, which is completely black, save a few dark-purple spots. Trees will turn into solid black background pillars and can no longer be chopped down.
Enemies spawned in The Shadows now do not include the Eater Family, but other existing enemies spawned in the Corruption will spawn there, with a few additional mobs.
Runs Unicorn AI. Attacks for 100 damage, has 60 defence and 300 health. Drops 1-5 Shadows upon death.
Will follow the player from a distance, is able to jump over ~10 blocks high. Creates dimensional rips near the player and causes objects to fall through them, damaging the player. Fallen objects deal 40-80 damage. The enemy deals 60 damage when touched, has 30 defence and 300 health. Drops 1-5 Shadows upon death, and has a 15% chance of dropping the Dimension Shard.
Follows Slimer AI. Has two forms, one with wings and one without. Attacks for 200 damage and has 60 defence in both forms, 50 health in the first form and 100 in the second. Drops 1-5 Shadows upon death together with Gel.
Flies overhead, burning anything in it's path with intense blasts of fire. This includes other mobs, together with the player. In it's first (flying) form, it deals 300 damage via it's fire, and 50 damage when touched. It also has 60 defence and 200 health. In it's second form, it falls to the ground and will attack by running into the player. It deals 80 damage when touched, has 30 defence and 200 health. Spawns semi-rarely, drops 5-10 Shadows upon death and has a 5% chance to drop Essence of Fire.
Spawns if the player has less than 100 health left. They appear in groups of 3 and follow the basic Fighter AI, but they move at a higher speed. They deal 100 damage, have no defence and 70 health. Drops 1-5 Shadows upon death.
Appears in packs of ~10 and follows the Fighter AI. Deals 100 damage and has 40 defence with 500 health. Spawns rarely and drops 1-2 Shadows upon death (each) and has a 5% chancee to drop the Shadow Harpoon.
Crafted with 5 Dimension Shards, a Gold Shortsword and 80 Shadows. Deals about 60 damage at a Fast speed, but attacks like a shortsword. Has a 20% chance for the opponent to stop moving entirely for half a second.
A sword crafted with 3 Dimension Shadrds, 2 Essence of Fire, 50 Shadows and one Adamantite Sword. Deals 50 damage at a Very Fast speed, but the player has a 10% chance each swing not to be able to move.
A gun crafted with 1 Megashark, 80 Shadows and 1 Dimension Shard. Deals 27 damage at an Insanely Fast speed and has a 80% chance not to consume ammo. However, it takes 10 Mana and 1 health to shoot every four bullets.
A harpoon weapon that does the same amount of damage as a ordinary Harpoon, but is able to fire two harpoons simultaneously. It takes 5 mana per shot and should your mana run out, it will fire one harpoon instead of two.