A friend of mine once had an idea for a mod that he would have really liked to be created, but over time it lost focus, and the coder vanished. I am reposting the request a few years later with a few additions from myself in hopes that someone might be able to take up this ambitious idea.
I put sections of it inside spoilers, because there's a fair bit, and a few pictures.
It starts by the player building an altar made of end stone and obsidian (So it's post-end). The player then lights the 4 obsidian pillars on fire (before the fires go out).
Once the last pillar is lit, all the fires turn black, and all the smoke will twist together into a ball above the center obsidian block. The player hears a voice whisper to them creepily, "What have you done...?" As the smoke morphs into the Rift Wraith, if but for a second before he explodes, destroying blocks in a large area, laughing eerily. The 'corruption' of the world has begun.
This biome continues spreading outward from where the altar once was, but it can only spread so far. If for some reason you wish to spread the darkness further, making more altars will accomplish that, but at a price.
The player must harvest these new blocks and build new tools. These new tools are the same strength as the tools you would normally have of the same tier, but only they can mine the new blocks. It is like starting Minecraft again, but darker. Black Matter torches can used to remove light from an area for when that is needed.
Creepers; When a Dark Creeper explodes it doesn't break blocks but instead sprays a damaging slime coating on the ground. This slime has a chance to spawn maggots when it dissipates naturally.
Zombies; Dark Zombies are stronger and always deal 1 heart of damage. When they are killed, a random number of maggots crawl from their corpse.
Skeletons; Dark Skeletons are extra nimble and avoid getting close to the player where possible. They have the speed equivalent of swiftness II, can jump 2 blocks high, and retreat when the player is within 4 blocks of them (Like an Ocelot running from a player).
Silverfish; You don't want to run away from a Dark Silverfish. They attempt to infest any Dark Stone nearby when the player is more than 5 blocks away and transform it into a maggot-nest. This block continually spawns maggots when you are close to it.
Witches; A Dark Witch can teleport around you, like an Enderman. Their potions are also of a higher tier. Avoid where possible.
Endermen; Dark Endermen become aggro when you go within 10 blocks of them, even through walls.
Spider; Dark Spiders are invisible by default, as well as damage increases and the ability to poison you makes them a larger annoyance. (Dark Cave spiders have a higher level poison)
Villagers; Dark Villagers in the area will trade Wraith marks for a variety of dark artefacts that could hinder or help your journey. Refusing to trade with one might make them mad however...
All Darkened passive mobs will attack you if you hit them, but because they're still just critters, they don't do much damage.
The Rift Wraith;
The Rift Wraith is the first miniboss you would encounter from this mod, but for every altar that is activated he gains more health and promptly vanishes back to the Shadow Rift.
The player must make a special altar to summon the Rift Wraith, and prevent him from returning if you wish to fight him. The altar can possibly look something like this, except the ore blocks are replaced with their corresponding Dark counterparts. (Iron = Shiveron, Gold = Wraith Essence, Redstone = Blood Powder, Diamond = Light Crystal).
Upon lighting the last pillar, the Rift Wraith spawns in the same fashion as with the previous one, except it does not explode because it cannot return to its dimension. Once it realises it is trapped (1-2 seconds), the eerie voice speaks to you, "You have sealed your fate mortal." and the fight begins.
The construct can only prevent the wraith from returning for so long, after 5 minutes he will return back to the Shadow Rift in whatever state you left it. To harm the Rift Wraith you must have a Light Crystal sword. The Rift Wraith has a base of 100 health which increases by 50 every time it is summoned, does 1 heart of damage each melee hit (no matter what armour), can teleport a short distance (like an enderman, just not as common), can cast an AoE blindness spell and can also cast the Drain effect where it steals some of your life to replenish theirs (the health stolen is scaled, 1 of your hearts heals it for 10 hearts (20 health)).
When the Rift Wraith is finally killed, it drops some Dark Crystal armour (random pieces, not all of it), a Black Matrix, and an Undecided Flame.
The Undecided Flame;
The Undecided Flame is a unique item that has only two uses. When thrown on the ground it will either Darken, or purify an area;
When you throw the Undecided Flame on the ground in Darkness and wait a short while, it will explode, and attempt to purify the Darkness into a random biome that seems appropriate (tall, thick trees = Jungle, lilypads+water = swamp, water = ocean/river (depends on depth), grass+trees = forest, grass = plains, tall grass = savannah, etc).
If the Undecided Flame is thrown on the ground in an undarkened biome, the Flame will explode and start to Darken the area like the altar would, except the Rift Wraith isn't spawned and thus doesn't gain HP.
Opening the Rift;
The Black Matrix dropped by the Rift Wraith is the keystone to entering the Shadow Rift. The player must make another special altar (salvaging materials from the previous one probably) and place the matrix in the center where the coal block is located. Here is a picture of what it can possibly look like (replace the mineral blocks with their Darkened counterparts as with the previous altar.)
The flames turn black as normal, the smoke centers in and around the matrix, engulfing it, then black lightning strikes the amassing ball of smoke tearing a hole in the fabric of space. The Shadow Rift.
The chunks around this Rift will always be Darkened and cannot be converted back unless the Rift is destroyed.
The Shadow Rift;
The Shadow Rift, the moment that the player has been preparing for all this time. It is a desolate place, filled with fragments of worlds that once were, but the wraiths have found it to be quite welcoming... The first impressions of the dimension may look like The End, floating islands separated by a fairly large distance...
But all biomes, all dimensions, Darkened and not, random sections of crumbling constructions (mountains, houses, giant trees, mansions, statues, etc) exist in this void.
As well as these, there exists new 'corrupted' biomes, where the wraiths have made their nests. The only mob you will find in this void is a variety of wraiths, and souls who were here before you and got trapped (mini bosses). These lost souls possess a variety of Dark Artefacts that are the only thing keeping them here.
Dark Artefact Mechanics;
Dark Artefacts store large and unimaginable powers that The Player can try to harness, but not everything is without its price. Dark Artefacts are powered by the void, and thus are at their full strength whilst in the Shadow Rift, luckily the artefacts possess a storage cell inside of them so the player may use them in the overworld, however greatly weakened, and limited uses. Either right clicking on the Rift Portal with an Artefact or possessing an Artefact on your hotbar whilst inside the Rift will 'recharge' the Artefacts, very slowly.
The more you use these artefacts, the more aggressive the wraiths become, spawning more frequently, stronger, faster. If you use them in the overworld, they might make an incursion to attempt to stop you. You are stealing the very embodiment of their home and bending it to your will after all.
As the player continues using the artefacts, the wraiths stop being just a generic wraith and become refined in special skills to combat against specific artefacts or weapons as well as attacking in groups to put their strengths together.
Eventually Greater Rift wraiths will spawn and it continues getting harder.
When enough is enough, the wraiths stop spawning altogether, the Shadow Rift becomes void of all life except you.
But its not over yet... Something is coming out of the darkness...
Possible Dark Artefacts;
Cloak of Shadows;
It is a billowing cloak that when placed in chest slot can render the user invisible. This invisibility is toggleable and when the Player is not invisible adds the ender particle effect around them. The cloak also absorbs a flat amount of 4 points of damage (2 hearts) when worn. If the player has a cape, the cape is rendered onto the cloak.
Artefacts of Power (Artefacts are unknown as to what they fully do and can be any of the below, The player has to test to find out what artefact they possess, categories are a convenience);
Damage everything within 3 blocks with a Dark Energy Pulse that disintegrates most mobs it comes into contact with (No drops)
Damage everything within 5 blocks with a Dark Pulse that if it kills something, disintegrates it.
Damage everything within 8 blocks with an explosion.
Fire a pulsating Dark Energy Ball that disintegrates most mobs.
Fire a Dark Ball that greatly damages mobs, and disintegrates if it kills.
Fire a Energy Ball that damages mobs.
Fire a small fireball.
Fire a snowball.
Launch an Energy Ball that creates a miniature vortex that sucks in most things around it removing them from existence.
Launch a pulsating Dark Energy Ball that on contact with anything releases a Dark Energy Pulse and disintegrates most mobs it comes into contact with.
Launch a Dark Energy Ball that on contact with anything releases a Dark Pulse and greatly damages most mobs it comes into contact with, it if it kills something, it disintegrates it.
Launch a Energy Ball that on contact explodes dealing damage to most mobs.
Launch a Fireball (Ghast fireball)
Summon a friendly wraith.
Artefacts of Deviousness;
All of the Power Artefacts, but with lifesteal and less damage.
Artefacts of Wisdom;
Various increased stats
Artefacts of Devastation;
All of the previous artefacts, but they deal random amounts of damage to you.
Explodes you, dealing damage to everything nearby, and greatly increased amount of damage to yourself.
If anyone could let me know if they could make, or at least start, this highly ambitious project, that would be wonderful.
If it is really needed, I could try and learn how to make models, but I would prefer someone that knows how to.