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.

After multiple warning messages (tentatively spanning hours) the army will appear. 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.

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
An open, flat space seems to work best for dealing with the Goblin Army, either on solid ground with no caverns or on an airborne platform. This allows you to easily deal with the fighters (they won't jump around and will be lined up for multiple hits) and help you get to the casters and dodge/destroy their missiles more easily.

Another good strategy is to 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. This gives you an easy and humorous way to deal with the army.

For another reusable strategy that uses a pit as well but uses fewer resources and requires further excavating into the landscape, do as follows.

Craft about 3-5 empty Buckets, preferably 10 to reduce the amount of trips. Dig to the Cavern layer to the small pools of lava. Collect the lava using the buckets by standing next to the pool. Return to the surface, a magic mirror is preferable to speed the ascent of this. Dig down 3 or 4 blocks down and 10-15 wide on both sides of the battle area. Fill the pit 1 block deep of the collected lava, this way the vast amount of the army is eliminated while the only enemy to fight is the sorcerers that will appear inside the area. The only downfall is the lost of common items such as coin rewards. You can still recover the higher rarity items that would have been droped like the Rocket Boots when covering up the lava with dirt blocks after the invasion. Also the battle would last 5-7 minutes, even less, depending on the amount of players at the time. Because this is a reuseable strategy when the next invasion is arriving the only thing to prepare for this battle is to uncover the lava again.