Goblin Army

The Goblin Army, also known as the Goblin Invasion, is a random event within the game when goblins will attack a player. The Goblin Army will spawn at a 6.666% chance (approximately 1 in 15) at dawn if you meet the criteria stated below.

When a Goblin invasion is imminent, a message will appear: This message repeats 5 times, and then changes to: Goblins will begin spawning at this point.
 * "A Goblin Army is approaching from the [east/west]!"
 * "The Goblin Army has arrived!"

The size of the army is based upon the count and health of the players within the game, according to the formula of 100+50x, with x being the number of players with a current HP of 200 or more. So, in single player, the army will consist of 150 goblins, while in an 8-player game, with each player having 200 HP or more, it will spawn 500. With the maximum of 255 players above 200 health, the maximum number of goblins in an invasion is 12,850.

In single-player, Goblins spawn in waves of 5-8 on-screen at a time. When the final wave of Goblins is on the screen, a message will appear saying, "The Goblin Army has been defeated!", and the theme music will return to normal.

Mobs
The army will be made up of the following goblins:
 * Goblin Peon
 * Goblin Sorcerer
 * Goblin Thief
 * Goblin Warrior

Criteria
The criteria for a goblin invasion to occur is as follows:
 * You must have at least one person in the game with 200+ current life
 * You must have smashed at least one Shadow Orb.
 * You must not have had an invasion within 7 in-game days.

Strategies

 * If you have the Vilethorn, you can simply remain in your house, barricade all doors with blocks (any type), and attack the Goblins as they come within range of your Vilethorn. You will be safe from all attacks except for the Goblin Sorcerer, however they will always teleport close enough to hit with the Vilethorn (even if not on their initial spawn), and the Vilethorn will also repel their projectiles easily.
 * On an open, flat space, you can easily deal with the fighters (they won't jump around and will be lined up for multiple hits) and help you get to the Sorcerer and dodge/destroy their projectiles more easily.
 * Dig a pit deep enough so the Goblin Army cannot jump out, and buy about 80 grenades. Toss the grenades into the pit, and use a Minishark or other ranged weapon to take out the Goblin Sorcerers.
 * A more complex strategy requires several buckets of lava, and a long, flat space. Choose a point roughly in the middle of this space; this is where you will stand during the invasion. On either side of this space, dig a trench 1 block deep 10-12 wide. Use your lava buckets to fill these trenches (three buckets for a twelve block trench is usually sufficient). After filling the trenches, test the lava depth by throwing a single copper coin into the lava. If your coin disappears, lengthen your trench by one block. Repeat the coin test until it no longer disintegrates in the lava. Now check if you have enough lava to cause damage; you can test this either with yourself (make sure you have at minimum 101 health) or by luring a monster in. Once your trenches are sufficiently filled, place two wooden platforms in the air several blocks above the trenches; make sure you can see the entire platform from your center space! If your setup is correct, the goblin army will spawn offscreen and die harmlessly in your trenches, the Goblin Sorcerers will spawn on the wooden platforms, wide open for a ranged assault, and the entirety of the loot will be waiting for you to pick up from the traps.