Bosses

Bosses are aggressive, resilient enemies that offer a substantial challenge to players. Defeating a boss is usually instrumental in advancing the game in some way. Each has its own particular way of being summoned. For example, most bosses have associated summoning items that can be used to spawn them manually under certain conditions, while there are others that will appear after the player interacts in a certain way with the environment. Event bosses will only appear during their respective events and leave when the event ends. Mini-bosses also exist and spawn naturally in various biomes under various conditions, which pose a lesser yet still difficult challenge for unsuspecting players.

The Calamity Mod expands on the number of bosses that can be challenged throughout the game, each featuring unique AI and drops. Most require the use of boss summoning items in order to spawn, but others have more unique prerequisites. The initial defeat of bosses (both vanilla and Calamity) typically results in some form of change within the world, be it new enemies appearing or a meteor crash, indicated by status messages to direct players towards the new content. Many also drop lore entries, usually explanations about what they are and why they exist. The mod currently adds twenty-two new boss encounters.

In Expert Mode, bosses drop Treasure Bags which contain powerful Expert-exclusive items in addition to their normal drops. However, many bosses also have new and unique attacks providing players with new challenges to surmount if they hope to reap the rewards. The Calamity Mod's new difficulty modes, Revengeance Mode and Death Mode, increase the challenge even further, often to levels that require rigorous training and complete knowledge of the boss fights if players hope to receive the valuable mode-exclusive rewards. There are other items that activate certain modes that can put the player to the test, further increasing the reward that the player can take, such as the Defiled Rune and Armageddon, while other items, such as the Iron Heart are for bragging rights.

Most bosses in Calamity are designed to be much more difficult than those of vanilla Terraria, offering a challenge even to seasoned players, but are not required to progress through the vanilla game's content. To make up for this, they often drop more powerful equipment and open up new tiers of progression. Any boss that is alive will inflict the Boss Zen debuff which reduces enemy spawn rates.

After the player has killed the Moon Lord, a whole new level of progression is unlocked, containing several very powerful bosses that will push the game's mechanics to their limits. These serve to test the player's skill and game knowledge and allow for additional tiers of weapons and items beyond what Terraria's endgame has to offer.

Unprepared players will find these bosses to be spectacularly difficult, but if proper preparations are made, all of them can be defeated at their respective level of progression. Players are advised to make use of arenas, potions and any other kind of aid they can muster before confronting these challenging bosses.

Vanilla Boss Additions
The Calamity Mod adds several differences to vanilla bosses ranging from drops to behavior in other difficulty modes.

Resistances
The Calamity Mod also modifies the amount of damage certain projectiles do to certain bosses.
 * All segments of The Destroyer take half damage from non-minion piercing projectiles. Additionally, they take:
 * 75% damage from the Spear of Paleolith's shards, the Desecrated Water's bubbles, the Kelvin Catalyst's stars and the rainbow trails from the Cosmic Rainbow.
 * 50% damage from the Dormant Brimseeker.
 * 38% damage from the Spent Fuel Container's explosions.
 * 25% damage from the Sea's Searing tornadoes.
 * All segments of the Eater of Worlds take 60% damage from non-minion piercing projectiles. Additionally, they take:
 * 70% damage from the beams from the Sparkling Empress.
 * The Lunatic Cultist takes 75% damage from the Resurrection Butterfly.
 * Duke Fishron takes 135% damage from the Resurrection Butterfly.

Boss summoning items
The Calamity Mod enables town NPCs to sell boss summons after that boss has been killed in the world. Most of the Pre-Hardmode summoning items are sold by the Dryad and most of the Hardmode summoning items are sold by the Witch Doctor.