This is not in the game or in a mod, this just fanon.
During hardmode, if it's a blood moon and you kill Skeletron Prime, you will get a message in chat. They will be sent every 2 minutes.
"An corrupted meteorite has landed."
"A terrible chill goes down your spine."
"Screams echo around you."
"You feel an large quake."
Once 5 Corrupted Meteorite have been mined, an gate will open in the center of Hell. When you go into the gate, you will enter the Dark Plane. It is composed of ebonstone, hellstone, adamantite ore and tiny amounts of the other ores. The trees are made out of petrified wood. 5 petrified wood mak an petrified sword, axe or pick. The petrified trees are as strong as obsidian. A petrified pick is as good as the Hamdrax but is slower. The petrified Axe has alot of knockback and chops down trees as fast as the Hamdrax, but is slow. The Petrified Sword gives you 5 armor, increases mana by 20, does 49 base damage and is fast. Once you start mining, you will notice new monsters coming, like Corrupted Pixies and Crazed Hellbats. Once you find the new ore underground, Corrupted Cobalt, you will be able to make the Cobalt Eye with 10 Corrupted Cobalt and 1 lense. When you use it, an message will appear in your chat.
"The Corrupted Eye of Cthulu has awoken!"
Then, an Eye of Cthulu will rush towards you, but it's purple, and it's shooting out vile spit. It has 40,000 hp, does 80 damage with each vile spit and does 60 damage if it touches you. After you get it down to 17,000 hp, it will turn into the Enraged Eye of Cthulu. It spits out 5 vile spit for 10 seconds, and does 70 damage if it hits you. But it won't charge at you anymore. Once it's finally dead, it will drop 6 Platinum Coins, a Damaged Pupil and a Corrupted Cobalt Helmet. The Corrupted Cobalt Helmet has 31 armor and increases ranged speed by 15%. The Damaged Pupil seems to be useless, but it opens up the portal back to your own world again. Then after that, with 50 Corrupted Cobalt, 10 Souls of Might and 20 Pixie Dust you can create the Chaos Rod. You craft it at the Mythril Anvil. It is a magic based weapon that has a base damage of 70. It can change colors from purple to gray and if you right click will spread Corruption. It costs 15 mana per shot. Once you enter the Dark Plane a second time, a message will appear in chat.
"Screams echo around you.."
After 10 minutes, another message will appear.
"The Nemean Lion has awoken!"
After this, an lion about 3 times the sixe of the player will charge at the player. It does 100 damage if it touches the player and is immune to all ranged damage. It has 10,000 hp and shoots out "Golden Shots" They are yellow missles that do 30 damage if you get hit. After you kill him, he will drop 6 Platinum Coins, Corrupted Cobalt Chainmail and a Damaged Mane. If you combine 100 Corrupted Cobalt, 10 Souls of Might, Damaged Mane and the Megashark you get the Nemean Assault Gun. It does 38 Base Damage, it's critical strike chance is 40, and gives you 10 Defense. It is Insanely Fast but it has no knockback. It only consumes ammo on the first bullet and has an 50% chance not to consume any at all. You craft it at a Mythril Anvil. The third time you enter, nothing will happen but a message will appear. The Corrupted Cobalt Chainmail adds 47 Defense and increases movement speed by 20%.
"The Dark Plane has been blessed with Corrupted Mythril!"
After this all the Iron Ore in the Dark Plane becomes Corrupted Mythril. 4 new enemies will arrive, the Goliath Zombie, the Goliath Slime, the Enchanted Drill and the Corrupted Leech. The Goliath Slime has 600 health and does 40 damage when it touches the player, the Enchanted Drill has 1000 hp and does 20 damage to the player , the Corrupted Leech does 50 damage and has 800 hp, but it is much longer then a regular leech and will split in two when an segment is killed. The Goliath Zombie has 1000 hp and does 40 damage when it touches the player. They all are really strong and will drop 1 Soul of Corruption. After mining 100 Corrupted Mythril, a message will appear in chat.
"Your world is in danger..."
After you come back from the Dark Plane, another message will appear.
"Corrupted Skeletron has awoken!"
Corrupted Skeletron has the same mechanics as Skeletron, but has 60,000 health and does 100 damage when it's hand touches the player and 150 with it's head spin attack. Once he has 10,000 health left he will start shooting Red Lasers at you which each do 50 damage. After it dies, it will drop 10 Platinum Coins, 20 Souls of Corruption and Corrupted Cobalt Leggings. The Corrupted Cobalt Leggings add 39 defense, increases movement speed by 20% and adds 50 hp. With 60 Corrupted Mythril and 40 Souls of Corruption, you can create the Tainted Lion Mane, which summons the Nemean Lion again. In this patch, there are new reforge enhancements. The 2 new ones are Corrupted, which decreases ranged speed and melee speed by 10% but increases damage by 25% and increases movement speed by 50%, and the other one is Superb, which increases everything by 2%.
After mining 20 Gold Ore in the Dark Plane, you will see a message in chat.
"Your world has been blessed with Hallowed Iron!"
Once you go to your world you will discover Hallowed Iron. It can only be mined with an drill with the Hallowed reforge. It increases size by 70% and allows you to mine Hallowed Iron. If you combine 10 Souls of Might, 10 Souls of Sight, 10 Souls of Sight, 10 Souls of Corruption, 50 Crystal Shards and 25 Corrupted Mythril you get the Flare Crystal. When used, it sends off a flare. It costs all mana and does 5000 damage. It gives you the "Tired" debuff. Tired makes you attack slower.
There are sets of armor you can make with the Hallowed Iron. With 15 Hallowed Iron you make "Hallowed Iron Greaves" which add 41 defense, increase damage by 20% and movement speed by 10%. The Hallowed Iron Helmet costs 10 Hallowed Iron and 5 Soul of Corruption. It has 23 armor, increases damage by 15% and increases ranged attack speed by 50%. The Hallowed Iron Mask, the counter part costs the same materials as the Hallowed Iron Helmet. It adds 13 armor, increases melee damage by 30%, increases movement speed by 10% and increases melee speed by 50%. The Hallowed Iron Chainmail adds 54 armor, increases hp by 100, increases ranged and melee speed by 10% and gives an 5% chance to get the buff "Talented" when killing a monster. It lasts 40 seconds and increases attack speed by 20% and gives 20 extra mana.
The final boss fight will be initaited after you ask Guide what to do next with full Hallowed Iron Armor. He will say "An unexpected surprise awaits you in the Dark Plane." When you go there, a sign will appear. It reads;
"Welcome to my lair. You thought Cthulu was controlling it all? No, it was me, the magnificent [Guide's name]. I've been plotting my revenge ever since you killed me by throwing me into lava... I made a deal with the Corruption, and became a GOD! Meet me in hell, and prepare to die. And if you don't come, I'll corrupt your world myself!!"
Every 3 days, the rate of corruption spreading will increase by 200% unless you fight Guide in hell. Once you go to hell, kill 10 monsters. Then, an message will appear in chat.
"Crazed Unholy Guide has arrived!"
The Crazed Unholy Guide will teleport next to you and begin the fight. Crazed Unholy Guide has 50,000 hp. He uses a huge war axe which if it hits you does 50 damage. He swings the axe fairly fast. After a few swing of the axe and running around, he wil fly up with Demon Wings and starts using the Nemean Assault Gun with Crystal Bullets. The bullet itself does 20 damage and the shard 5, but the Nemean Assault Gun goes slower then it's real stats, and only goes as fast as the Minishark. After firing 15 bullets he will come down with a Petrified Sword and if it hits you does a whopping 200 damage and knocks you back alot, even if you have a knockback cancelling acessory. But his sword gets stuck in the ground for 10 seconds and you deal 40% more damage during this time. When he pulls it out he will make a huge swipe around him, dealing 70 damage. Then he repeats. After he dies, text is shown on the chat.
"[Guide's name]: How can this.... be...? I was invincible.... blessed by the great Unholy ones......... You... Cthulu will kill you... "
After this, Guide's sprite returns to normal and says
"[Guides name]: Thank you... the Unholy Ones corrupted me... But here, have this as a reward. I have no use for it anymore."
He then throws infront of him 20 Platinum Coins, 5 of each soul and Guide's Breaker Scythe. The Guide's Breaker Scythe is a spear that does 91 damage, is Average speed, increases movement speed by 20% and increases the spawn of monsters in the Hallow. When you right click, it will consume 10 mana and it fires out an 3 shot cursed flame. Each flame does 30 damage.