In Terraria, defeating bosses is a crucial part of the game progression. Each boss can typically be summoned with specific items, which players can craft or find within the world. Here’s a guide to the items needed to summon each boss and how to obtain them:

Pre-Hardmode Bosses

1. Eye of Cthulhu
   • Summoning Item: Suspicious Looking Eye
   • How to Craft: Crafted at a Demon/Crimson Altar with 6 Lenses (dropped by Demon Eyes during nighttime).
   • Natural Occurrence: Has a chance to spawn naturally if a player has at least 200 HP and at least 10 defense, and three NPCs are housed.
2. Eater of Worlds
   • Summoning Item: Worm Food
   • How to Craft: Crafted at a Demon Altar with 30 Vile Powder (made from Vile Mushrooms) and 15 Rotten Chunks (dropped by Corrupt enemies).
   • Natural Occurrence: Break three Shadow Orbs in the Corruption.
3. Brain of Cthulhu
   • Summoning Item: Bloody Spine
   • How to Craft: Crafted at a Crimson Altar with 15 Vertebrae (dropped by Crimson enemies).
   • Natural Occurrence: Break three Crimson Hearts in the Crimson.
4. Queen Bee
   • Summoning Item: Abeemination
   • How to Craft: Crafted anywhere with 5 Honey Blocks, 1 Stinger, 5 Hive, and 1 Bottled Honey.
   • Natural Occurrence: Break the larva inside Bee Hives found in the Underground Jungle.
5. Skeletron
   • Summoning Item: None (summoned by talking to the Old Man at the Dungeon entrance at night).
   • Natural Occurrence: Speak to the Old Man at night at the Dungeon entrance and choose the curse option.

Hardmode Bosses

1. The Twins
   • Summoning Item: Mechanical Eye
   • How to Craft: Crafted at a Mythril or Orichalcum Anvil with 6 Lenses, 5 Iron or Lead Bars, and 3 Souls of Light.
2. The Destroyer
   • Summoning Item: Mechanical Worm
   • How to Craft: Crafted at a Mythril or Orichalcum Anvil with 6 Rotten Chunks or 6 Vertebrae, 5 Iron or Lead Bars, and 6 Souls of Night.
3. Skeletron Prime
   • Summoning Item: Mechanical Skull
   • How to Craft: Crafted at a Mythril or Orichalcum Anvil with 30 Bones, 5 Iron or Lead Bars, 3 Souls of Light, and 3 Souls of Night.
4. Plantera
   • Summoning Item: None (summoned by breaking a Plantera’s Bulb, which spawns in the Underground Jungle after defeating The Destroyer, The Twins, and Skeletron Prime).
   • Natural Occurrence: Break Plantera’s Bulb in the Underground Jungle.
5. Golem
   • Summoning Item: Lihzahrd Power Cell
   • How to Obtain: Found in Lihzahrd Chests in the Jungle Temple or dropped by Lihzahrd and Flying Snakes within the temple.
   • Usage: Used at the Lihzahrd Altar in the Jungle Temple to summon the Golem.
6. Lunatic Cultist
   • Summoning Item: None (summoned by defeating the cultists that appear at the Dungeon entrance after Golem has been defeated).
   • Natural Occurrence: Kill the cultists at the Dungeon entrance.
7. Moon Lord
   • Summoning Item: Celestial Sigil
   • How to Craft: Crafted at an Ancient Manipulator with 20 of each Celestial Fragment (Solar, Nebula, Stardust, Vortex).
   • Natural Occurrence: Automatically spawns after defeating the four Celestial Towers from the Lunar Events.

Understanding these summoning items and the conditions for each boss allows you to effectively plan your battles and progress through Terraria’s extensive content. Make sure you’re adequately prepared with weapons, armor, and potions to tackle these challenging encounters!