Progression Stages (2.77.5)

"'Your actions have consequences.'"
 * Rebirth of the Night's progression is divided into various stages that trigger once a condition is met.
 * These conditions include obtaining a specific item for the first time, obtaining certain sets of armor and weapons, entering a new dimension for the first time, defeating a boss for the first time, and more.
 * It is worth noting that ANY of these mobs can very very rarely spawn no matter what, even without any stages unlocked. You may see a Fire Skeleton, for example, even if you didn't enter the Nether, though this is extremely rare.

Multiplayer
Some progression stages are global, affecting all online players, some are not. Personal growth stages (early game armor and mid-game armor) only affect the player who got the achievement. All other stages affect the entire world, meaning all online players. If you are playing on a server and not teaming up with players, it's recommended that you advise them to log off while you complete a progression stage, so that newer players do not unlock them. Obviously, this only matters if the players are far away from each other.

If a player logs on for the first time after all the stages are unlocked, they won't have any of the progression stages, meaning progression mobs won't spawn around them if other players are far away. They can still unlock all stages themselves.

Early-Game Armor
"'You feel uneasy, even as you grow in power...'" Triggered when the player obtains a full set of pre-Diamond armor (bronze, iron, silver, etc, but not leather)

Effects:
 * Invasions
 * The skeleton horde is approaching...
 * The spider horde is approaching...
 * The zombie horde is approaching... (also occurs before any progression stages!)
 * All Dimensions
 * Zombies, skeletons and skeleton warriors now have a chance to spawn with bronze armor
 * Overworld and Beneath
 * Hungry Zombies now spawn
 * Sniper Skeletons now spawn
 * Hungry Spiders now spawn
 * Pale Spiders now spawn
 * Rocket Creepers now spawn
 * Overworld
 * Wind Witches now spawn
 * Stymphalian Birds now spawn
 * Pale Lurkers now spawn
 * Penghouls now spawn (very cold biomes)
 * Pteras now spawn (hot and ocean biomes)
 * Vespas now spawn (hot biomes)
 * Brain Slimes now spawn (ocean and beach biomes)

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Mid-Game Armor
"'The ground shudders as you sense something stir deep beneath the world...'" Triggered when the player obtains a full set of mid-game armor (diamond, ruby, obsidian, etc).

Effects:
 * Invasions
 * The pirates are invading!
 * The creeper horde is approaching...
 * The undead cavalry rides tonight!
 * You hear a loud roar coming from the skies...
 * You hear a flock of birds in the distance...
 * The average spider eats eight humans every year...
 * A goblin army is invading!
 * You hear buzzing and slithering in the distance...
 * All Dimensions
 * Zombies, skeletons and skeleton warriors now have a chance to spawn with silver and iron armor
 * Zombies and goblins now have a chance to spawn with iron and bronze pickaxes
 * Overworld and Beneath
 * Brute Zombies now spawn
 * Brute Skeletons now spawn
 * Jumping Creepers now spawn
 * Witch Spiders now spawn
 * Flying Silverfish now spawn
 * Scarecrows now spawn
 * Gluttonous Stalkers now spawn
 * Overworld
 * Grape Slimes now spawn
 * Wild Witches now spawn
 * Dire Wolves now spawn (cold biomes)
 * Desert Spiders now spawn (desert biomes)

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Entering the Nether
"(warning) 'The air around you begins to burn...'"

"'Cries of the damned echo around you...'" Triggered when the player enters the Nether for the first time.

Effects:
 * Invasions
 * You better BEE prepared tonight...
 * Fire burns on the horizon...
 * A horde of monsters has stormed out of the Nether!
 * All Dimensions
 * Zombies, skeletons and skeleton warriors now have a chance to spawn with gold armor
 * Overworld and Beneath
 * Magma Monster Grunts now spawn
 * Living Embers now spawn
 * Fire Zombies now spawn
 * Fire Creepers now spawn
 * Fire Skeletons now spawn
 * Overworld
 * Salamanders now spawn
 * Strawberry Slimes now spawn

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Entering the Aether
"'The wind howls, carrying a dread cold...'" Triggered when the player enters the Aether for the first time.

Effects:
 * Overworld
 * Kelenk now spawn
 * Lemon Slimes now spawn
 * Aether
 * Pteras now spawn
 * Skydrakes now spawn

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Entering the Beneath
"(warning) 'The barrier trembles. You sense a foul presence yearning to break free...'"

"'Shadows squirm and writhe around you...'" Triggered when the player enters The Beneath for the first time.

Effects:
 * Invasions
 * A swarm of bugs is approaching...
 * They're the rats! And all of your base belongs to them!
 * Hundreds of tiny wings flap in the distance...
 * You hear a haunting flute melody growing louder...
 * Overworld and Beneath
 * Plague Zombies now spawn
 * Dark Creepers now spawn
 * Undead Witches now spawn
 * Blinding Silverfish now spawn
 * Poison Silverfish now spawn
 * Zotzpyres now spawn
 * Overgrowns now spawn (warm biomes)
 * Beneath
 * Death Creepers now spawn
 * Overworld
 * Rats now spawn
 * Poison Spiders now spawn
 * Scarecrows now spawn (spooky biomes and forests)
 * Dread Swines now spawn (forests and warm biomes)
 * Frostbounds now spawn (cold biomes)
 * Aether
 * Coolmar Spiders now spawn in the Aether
 * Harpies now spawn in the Aether

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First Mythril
"'The darkness erupts with distant screeches as the last magic barrier shatters...'" Triggered when the player mines Mythril ore for the first time.

Effects:
 * Invasions
 * An Illager army is invading!
 * The animal uprising has begun. They want revenge!
 * OOPS! All Vermin Souls!
 * The legendary undead is approaching...
 * They prey at night... They stalk at night...
 * The witch covens are invading!
 * This is going to be a terrible night...
 * All Dimensions
 * Zombies, skeletons and skeleton warriors now have a chance to spawn with diamond and obsidian armor
 * Zombies and goblins now have a chance to spawn with diamond pickaxes
 * Overworld, Beneath, Nether, and End
 * Corrupted Endermen now spawn
 * Overworld, Beneath, Nether
 * Vermin Souls now spawn
 * Overworld and Beneath
 * Fishing Zombies now spawn
 * Lightning Creepers now spawn
 * Ninja Skeletons now spawn
 * Web Spiders now spawn
 * Shadow Witches now spawn
 * Plague Beasts now spawn
 * Ithaquas now spawn (cold and forest biomes)
 * Overworld
 * Black Death (boss) now rarely spawns in the Overworld
 * Blueberry Slimes now spawn
 * Osvermis' now spawn (warm and spoopy biomes)
 * Aether
 * Wights now spawn
 * Frosts now spawn

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Rebirth of the Nether (Post-Wither)
"'Voices of lost souls scream in fury...'" Triggered when the player defeats The Wither.

Effects:
 * Nether, Beneath, and Overworld
 * Blightspore Creepers now spawn
 * Nether and Beneath
 * Hardened Magma Slimes now spawn
 * Nether
 * Increased Magma Monster Grunts spawn rate
 * Blazes now spawn anywhere
 * Blaze Juggernauts now spawn
 * Withering Spine now spawn
 * Magma Monsters now spawn
 * Brute Pigmen now spawn
 * Hungry Pigmen now spawn
 * Doom Creepers now spawn
 * Flying Magma Slimes now spawn
 * Sticky Magma Slimes now spawn
 * Cinder, Ember, Hellfire, Inferno, and Jolt Blazes now spawn
 * Brute, Giant, Sniper, and Spitfire Wither Skeletons now spawn (Nether Fortresses)

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Rebirth of the World (Post-Ender Dragon)
"'Fragments of a lost empire echo around you...'" Triggered when the player defeats The Ender Dragon.

Effects:
 * All Dimensions
 * Zombies, skeletons and skeleton warriors now have a chance to spawn with mythril and viridium armor
 * Zombies and goblins now have a chance to spawn with mythril and onyx pickaxes
 * Overworld, End, and Beneath
 * Void Eyes now spawn
 * Overworld and Beneath
 * Giant Zombies now spawn
 * Giant Skeletons now spawn
 * Giant Spiders now spawn
 * Ender Creepers now spawn
 * Spitfire Skeletons now spawn
 * Flying Spiders now spawn
 * Domination Witches now spawn
 * Tough Silverfish spawn
 * Overworld
 * Mirage Enderman now spawn
 * Watermelon Slimes now spawn
 * Icy Enderman now spawn (cold biomes)
 * Lightning Enderman now spawn (mountain biomes)

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Rebirth of the Forest (Post-Queen Ghast) (DISABLED UNTIL 3.0)
"'Visions of mythical beings flash through your mind...'" ''Triggered when the player defeats The Ghast Queen. ''(or craft the queen ghast tear in 2.77)

Effects:
 * Overworld and Beneath
 * Gatling Skeletons now spawn
 * Spitfire Wither Skeletons now spawn
 * Mother Spiders now spawn
 * Splitting Creepers now spawn
 * Festive Creepers now spawn
 * Trollagers now spawn
 * Nether
 * King Ghast now spawns
 * Splitting Ghasts now spawn
 * Conflagration Blazes now spawn
 * Wild Fire Blazes now spawn
 * Sticky Magma Slimes now spawn
 * Volatile Magma Slimes now spawn
 * Overworld
 * Blackberry Slimes now spawn
 * Shadow Witches now spawn
 * Flying Silverfish now spawn (ocean and river biomes)
 * Beneath
 * Tormented Ghasts now spawn (yeah, I know)

Trivia

 * The status messages seen in the chat are inspired by Terraria.
 * This feature used to be on a difficulty meter.
 * Invasions will be tied to these stages in 2.77
 * In a future update, all zombie and skeleton-based mobs will have access to the upgraded armor tiers.