In Terraria, various events occur that can dramatically change the gameplay experience. These events range from invasions by enemy armies to unique environmental occurrences that challenge players with new enemies and provide opportunities to collect exclusive items. Here’s a timeline and guide on when and how these major events can occur in your Terraria adventure:

Pre-Hardmode Events

1. Blood Moon
   • When: Can occur randomly at night once a player has at least 120 health.
   • What Happens: The moon turns red, and enemy spawn rates increase significantly. Certain enemies like the Blood Zombie and Drippler appear, and NPCs sell unique items.
2. Goblin Army
   • When: Can occur randomly after at least one player in the world has over 200 health and at least one Shadow Orb or Crimson Heart has been destroyed.
   • What Happens: A large army of goblins invades, consisting of various goblin enemies.
3. Slime Rain
   • When: Can occur randomly once a player has at least 140 health.
   • What Happens: Slimes rain from the sky, culminating in the King Slime boss battle if enough slimes are killed.
4. Eye of Cthulhu
   • When: Can be summoned at night using a Suspicious Looking Eye, or it may appear naturally once a player has at least 200 health and at least three NPCs.
   • What Happens: A boss battle with the Eye of Cthulhu.

Hardmode Events

Hardmode begins once the Wall of Flesh is defeated in the Underworld.

5. Pirate Invasion
   • When: Can occur randomly after the Wall of Flesh has been defeated, or can be summoned using a Pirate Map.
   • What Happens: Pirates invade your world, including various pirate-themed enemies and the Pirate Captain.
6. Solar Eclipse
   • When: Has a chance to occur naturally each morning after at least one Mechanical Boss has been defeated, or can be summoned using a Solar Tablet.
   • What Happens: A daytime event where monsters inspired by horror movies appear, such as Mothron, Reapers, and Vampires.
7. Frost Moon
   • When: Can be summoned at night using the Naughty Present, which is craftable during the Christmas season.
   • What Happens: A wave-based event with Christmas-themed enemies, getting progressively harder with each wave.
8. Pumpkin Moon
   • When: Can be summoned at night using the Pumpkin Moon Medallion, which is craftable during the Halloween season.
   • What Happens: A Halloween-themed wave event featuring various monsters like the Mourning Wood and Pumpking.
9. Martian Madness
   • When: Can occur after the Golem is defeated if a Martian Probe spots a player and escapes.
   • What Happens: Martians invade, bringing advanced technology and unique enemies like the Martian Saucer.

Seasonal Events

• Halloween (October 20th — November 1st): The world takes on a Halloween theme, where enemies drop Goodie Bags, and Pumpkins grow naturally.
• Christmas (December 15th — December 31st): The world takes on a Christmas theme, enemies drop Presents, and NPCs wear Christmas attire.

Event Handling Tips

• Preparation: For each event, prepare your arena with ample space, good lighting, and layers of platforms for maneuverability.
• NPC Safety: Place your NPCs in homes away from your main arena to protect them during invasions.
• Gearing Up: Equip armor and weapons suitable for dealing with large groups or tough bosses. Use buff potions to enhance your abilities during these events.

Understanding when and how these events occur can help you better prepare and take full advantage of the unique opportunities they provide. Whether for gearing up, farming for items, or simply enjoying the varied gameplay, each event adds depth and excitement to the world of Terraria.