The next blood moon after killing all the hardmode bosses, if you are wearing any type of Hallowed Armor, you will suddenly see a few sentences appear in the chat, one after the other:
"You feel an evil presence watching you..."
"A horrible chill goes down your spine..."
"Screams echo around you..."
"You sense an unsettling presence coming somewhere far below the surface."
This means that the Hell Keeper has spawned somewhere within 1000 feet of the center of the map, in the Underworld. Every blood moon, if not defeated it will move to a new location, killing a random NPC in the process.
It will be invisible until you damage it, meaning that there will be no way to find the Hell Keeper without using a Hunter's Potion, or via pure luck.
Once attacked, you will be inflicted with the debuff "Demon's Wrath", which sends you 1/8th of the map away. The demon will become visible, and behind him, the Demon's Gate appears (More on that later).
The Hell KeeperEdit
A few years before you arrived in the world of Terraria, the Demons were unsatisfied with the protection their world gave them(Yes, the world used to belong entirely to the Demons). They didn't trust in the might of the Wall of Flesh to keep the Hallow and Corruption behind bars, and knew that sooner or later someone would release them.
They decided to formulate a plan to expand the Underworld in which they ruled, and realised that making an army in secret would be the best. They then created a gate to another dimention in the Underworld, and summoned Keepers to protect it from dangers before they were ready.
Threats came sooner than expected. An unknown person managed to find the secret behind the Guide, and destroyed the Wall. The world was thrown into chaos with the Hallow suddenly turning hostile to keep itself from being diminished by the corruption. With all the other Keepers (Bosses, Sk prime, etc) Killed, the gate was suddenly prone to attacks, as the other Keepers lent their souls to keeping it hidden.
They sent the last of the Keepers, the most respected fighter in the Demon community, to guard the gate. However, he wasn't strong enough to protect the gate entirely and resorted into hiding it, constantly moving it under the cover of the blood moon.
The Hell Keeper has the basic AI of the demon, but moves twice as fast and has the ability to teleport if any ranged attacks are approaching, including Boomerangs. He has a ten second cooldown between 3-in-1-second teleport bursts, so its easier to hit him.
He has 200,000 health and takes about 25% less damage from spells.
Pointing his staff at you, he can shoot Kamikaze Bombs, which look like the Destroyer's Probes but after shooting a laser, it rams into you, exploding to deal 60 damage (Without armor). They can't pass through walls.
Sometimes, he throws out five Hell Bombs, which explode on impact, dealing 20 damage each, disregarding armour. He only does this when you have been standing still for more than half a second and he is over you.
He also can swing his staff, bringing a collumn of fire up from open pits of lava to inflict 30 damage (without armor) at a fast rate, inflicting the "On Fire!" debuff. When there are blocks hindering the explosion, the blocks will be "mined", and they will drop into the lava to be burnt.
Once in a while, he will suddenly teleport to a flat area of ground (At least 3 blocks wide) and slam the bottom of his staff on the ground. There will be a message saying "A legion of the Hell army has been summoned!" And many hellbats and skeleton archers will approach from each side of the screen.
Once at a quater of his health (50,000 health), he swings his staff and you are inflicted with the "Judgement" debuff. Enemies cannot spawn, and you can't move, but neither can the Hell Keeper. After ten seconds, everything returns to normal and depending on whether there is more hallow or more corruption, what happens might be different.
More hallow: Boss moves at twice speed and health returns to 70,000. Judgement cannot occur again.
More corruption: Boss moves at half speed.
If the player dies while fighting the boss, the boss will move to a new location.
The boss will drop the Demon tablet.
When Summoned "You have brought this upon yourself."
When Summoned "You have made a bad descision, mortal."
After five minutes "Why do you persist?"
Once at half life "Enough of this nonsense!"
Sometimes when teleporting "You can run, but you can't hide!"
At 50,100 health (Right before Judgement) "No! I shall protect the Demons from your insane ways!"
Judgement, More hallow "Why are you siding with the Hallow? It brings nothing but pain!"
Judgement, More corruption "Siding with the strong, eh?"
Player dies "And stay out!"
The Demon's GateEdit
The Demon tablet can be crafted together with one book and a golden key at an Angel Statue (Finally it does something!) to make the Magic Key.
This key can be used to open the Demon's Gate. Upon opening it, a large explosion (Which deals 99999 damage) insta-kills you. You do not drop any items, even if you are in Mediumcore or Hardcore mode.
Once you respawn, the world will enter the Dark Mode with the quote "The Hell Keeper has been slain and the Demon's Gate opened. Hell has risen!"
As the Gate was opened before the army was finished, the Hell army was not able to claim the entire world. Therefore, instead of the entire world becoming hellish, Corruption and Hallow can no longer exist below -300 feet, at which point underworld enemies will spawn.
All new Hell/Corruption based enemies have health based on Base health x Corruption%/Hallow%
Flies twice as fast as the normal Demon and fires off the Demon Scythe twice as fast, too. Has a base health of 350 and deals 50 damage.
Shoots out Flaming Symbols at random speeds very quickly at the player, much like how the Magical Harp shoots out music notes.
These symbols go through blocks and attacks until 30 blocks from the player, where they start bouncing off walls. The symbols do 30 damage and inflict the "On Fire!" debuff 60% of the time. They have 2 health and 999 defence.
The flamers themselves have the same stats as the Fire Imp, except that their health is affected by the corruption.
It has one health and does 5 damage, regardless of health. They have the Worm AI and turn all blocks they touch (other than bricks and walls) into ash. Drops Flaming Snakeskin.
Shoots Lava Bombs with bows. These bombs turn into half a block of lava upon impact. The Hell Troop deals 100 damage when touched and has 200 base health.
Has the abillity to summon Hell Troops. Deals 100 damage when touched.
Has two heads. One of them tries to latch on to you while the other tries and turns around to shoot Vile Spit at you. The result is a spazzing enemy which follows you everywhere, shooting Vile Spit in all directions. Deals the same damage as a Corruptor and has 400 base health.
Power Seeker Has the same stats as the Chaos Elemental, except that this enemy can teleport right onto you and heals the damage it deals you.
Has 1/10 chance to replace a Power Seeker. It spawns Illuminant enemies every second around your screen until you kill him or the max enemy limit is reached. Spawns more often when near corruption as well. Deals 60 damage when touched.
Close range weapons deal 50% more damage when the player has equipped the Titan's Glove.
Ten times the size of the player and spawns on a blood moon when the Demon's Gate is opened again (The key is reusable). This enemy shoots Hell Bombs at the player while Undead Firesnakes fall from the ceiling. Once in a while, the enemy will send out a few Hell Super Soldiers which are enemies who follow the Fighter AI, but attempt to stay at least 6 blocks away from the player. They carry flamethrowers and wear the Adamantite ranged set. They have 300 Base Health and deal basic Flamethrower damage.
This boss has 60 defence and 200,000 health. It doesn't have a time limit so you can take as long as you want to kill it.
When killed, it will drop the Soul of Height, which allows crafting of the Mechanical Bait (More on that later), and the Inferno sword.
Crafted with Ten Soul of Height, twenty Flaming Snakeskin (You probably get at least ten from those falling during the battle with the him) and the Breaker Blade at the mythril anvil. This weapon is on-par damge-wise with the Excalibur, but it is much larger and slower, and summons Fiery waves (At the cost of 50 mana) with each swing. If you run out of mana, the Inferno can still be used, but there will not be any special effects. The wave does Melee damage.
Spawns as an NPC in your house after the Demon Gate is opened. The NPC doesnt say anything other than "......" when you talk to him, and doesnt offer any services.
Two days after the next Blood Moon, if he is present, he will suddenly be gone from his room. Once night arrives, The message "The blue moon is rising..." Will appear. When you talk to The Shaken during this new event, all the NPCs will suddenly die (Yeah, wtf?) and "The Shaken has awoken!" will appear.
The Shaken (Now a boss) will be a shadow that only appears when you are close to it or when you damage it. It inflicts you with the "Shaken" debuff when you touch it, instead of dealing damage to you directly. The debuff causes you to not be able to move, and you will slowly lose health. You can still shoot The Shaken, though. Once it has been hit a hundred times, it will transform into Skeletron Prime's head with half health. Once defeated, The Shaken will then resume the form of the shadow. This is repeated with the Destroyer and The Twins. Once defeated, The shadow will fade out and a NPC, the Encouraged, will appear. Of course, like other NPCs, this one will have a name before it.
Encouraged will have a option to "Bring it on!" which will inflict the player with a random buff. Buffs can only be provided one at a time, and if these buffs are removed,there will be a debuff equal to the time which was left that prevents you from getting any more buffs. Encouraged will not be able to buff you during a blood moon.
"I see a bright future ahead!"
"Why dwell on my past?"
"Give me a [First letter of player's name]!"
"Let there be light!"'' (Between 4am and 5am in-game)
"Its SUNday everyday!" (During the day)
"Its like the darkness in me has been slain!"
"Wha-WhaaAGAHHHHH!" (During a Blood Moon)
"GET AWAY FROM ME!" (During a Blood Moon)
"NOT THIS AGAIN!" (During a Blood Moon)
"......" (During a Blood Moon)
The final boss of the Dark mode.
After it is defeated, the world will tremble and the world will be returned to Hardmode, but with the hallow not being as agressive as before, and the corruption and hallow spreading at half-speed.
The Destructonator can be summoned by throwing Mechanical Bait at the Demon's Gate. This is crafted with twenty ash, dynamite, fifty wire, ten copper and iron, twenty of each soul (other than flight), fifty crystal shards and fifty cursed flame. The Mechanical Bait is thrown like a grenade, and will detonate upon touching the gate. If the it doesn't come into contact with it by the time it stops moving, it will turn back into an item which will be able to be picked up. It is not destroyed by Lava.
The Destructonator will emerge from the Demon's Gate in a poof of smoke, breaking it in the process (Its HUGE), and start moving toward the player. The Destructonator is able to fly and teleport, though it only teleports if the player is a thousand feet away.
Touching the Destructonator will instant-kill you, but during the fight with him, he will follow you wherever you go, even if you die. Depending on the biome you are in, his attack pattern will change. If you are in multiple biomes at once, the Destructonator will gain both abilities.
The boss can spew out Hell Bombs from its chest, as well as Hell Commanders.
The boss can turn around, having it's blades facing the player, it can then spin them to create a sand-storm which deals 20 damage (regardless of defence) per second to the player until it's blades are hit 10 times with a melee weapon, or 20 times with a ranged weapon.
The boss will start spawning Dungeon Guardians, so its best to avoid this area.
Ocean (Also includes Desert if you are not in the water)Edit
The boss points it's water cannon on it's right arm at the player, spraying out high jets of water to deal 40 damage per second (20 per half-second) to the player. This will continue until either the player either goes out of the stream or the boss runs out of water. The water jets deal knockback enough to stun-kill a player without the Cobalt Sheild.
The boss will pull out it's machinegun on it's left arm and start firing at the player. This is almost-always instant-kill, as it does 240 damage per second (60-60-60-60) as it's bullets are explosive. However, the machinegun does take a second to start spinning so its possible to get out of the way before you get obliberated.
The boss will fire lasers from it's eyes which deal 50 damage. These lasers also turn the area they hit into a time-bomb which explodes after a second. This explosion will not damage blocks, but will deal massive damage to the player. Killing the Destructonator also unlocks a new enemy: The Healer. This enemy self-destructs if touching a corrupt enemy, and does not attack the player unless attacked. It takes the form of a large green pixie. When you kill it, you will recive twice as much Fairy Dust than if you just killed a normal pixie. You can access the Dark Mode anytime by killing the Hell Keeper again (It respawns after the next two bloodmoons, but when it moves, your NPCs no longer die.)
Demon armour setEdit
Hell Keeper's Helmet
Crafted from the Shadow Helmet, Molten Helmet, an Obsidian Skull, ten Soul of Height and ten Soul of Might. You are invincible to the "On Fire!" debuff, and lava deals half as much damage. You do not recive damage from Hellstone Ore and Meteor Ore.
Crafted from the Shadow Scalemail, Molten Breastplate, a Cobalt Sheild, ten Soul of Height and ten Soul of Might. You do not recieve knockback.
Crafted from the Shadow Greaves, Molten Greaves, The Cloud in a Balloon, ten Soul of Height and ten Soul of Might. It allows you to Double Jump like the Cloud in a Bottle, except that the second jump brings you as high as the Cloud in a Balloon can bring you. The cloud effect is replaced by an exploding effect.
None of these items grant you any defence bonus. However, they are equipped in the Social slot, but add on to the stats of the armour in the armour slot. They different set bonuses when equipped with different sets of the Hallowed Armour, and it cannot be seen and the effects don't work if you are not in the Dark Mode.
Set Bonus (Melee)
Invinciblility period increased by half a second.
Set Bonus (Ranged)
When dead, you respawn with full health.
Set Bonus (Magic)
The Demon Scythe spell now can be used twice as fast.