THE ENTRAYLS MOD
You've been looking for so long, so long in fact, you've forgotten what you were looking for in the first place. Well, forget it if you just remembered. The End is just the beginning of the mysterious Builders; as for what they left behind? Well, let's tinker with one of their last crafts: the Entrayls.
What are Entrayls?

Entrayls are beings made by a secretive few of the old Builders, before they all vanished. They are given one purpose in life: capturing Endermites in special containers, known as Abysl Frames, and using magic to transform them into Abysl, a raw material that can be used to mass-produce a select handful of items found throughout your travels.
Once you get an Eye of Ender, this mod's events will begin to take place.
Entrayls' workstations, also known as Craftboxes, will have a slim chance to appear in the Overworld; your best bet would be to visit the Desert, Badlands, or Snowy Plains to see if you can catch an appearace. You'll know one's around if a lightning strike occurs in the distance. Endermites will levitate from the Craftbox like clockwork, and shortly after, Entrayls will pop out to begin the process. Unfortunately for them, the Builders are no longer around, so as long as they have access to Endermites, Entrayls will continue to stack Abysl around the Craftbox.
You can harvest the Abysl if they happen to fall as an item, or if mined with a Diamond Hoe, but be very cautious when doing so. Entrayls will defend the Craftbox by throwing half-baked Abysl, or Abyslites, towards you dirty thieves. Entrayls are weakened in damage and resistance by using a fishing rod on them; especially useful if they keep hovering out of reach!
The Entrayls are mysterious beings, so you will not find them in the End, no matter how hard you look. Killing them will provide a Phantom membrane... ew.
Abyslites: Ticking Time Bombs

Abyslites are often used as fodder for Entrayls, and when they spot a player, they'll rush towards them. They'll even move sporadically to avoid being hit while making their way to you. If they do reach you, or if their time in the Overworld runs out, they'll explode with a power of 1; killing them will yield a power 2 explosion.
Abyslites can spawn naturally in the End, and behave slightly different: they will hover aimlessly until you get close enough, then rush you down to explode. Unlike existing in the Overworld, they will not explode after some time has passed, so you had better outpace them, or shoot them before they get close enough.
If the Entrayls can build them, theoretically Builders could, as well. There are two ingredients required to create an empty Abysl Frame: 8 End Rods around a soul (but we'll go over that soon, don't worry). When you have a frame in your main hand, find an Endermite and attack it to capture it, converting it into an Abyslite. Then, find enough room, and right-click the air to release the Tameable Abyslite.
Yes, that's right! Taming it is very simple, given the process to create one is complex: sneak up to it, and hit it! I know that sounds harsh, but they actually enjoy the contact, marking you as their owner for life. Strange little End beings, they are; they'll be healed half a heart for every knock on the noggin, as long as you're sneaking and have no weapons equipped. If you have dyes in your main hand, and you sneak-hit the Abyslite, you can color it without consuming the dye. That, however, is only half of the charm.
They'll follow you everywhere you go, and even have a small inventory! That's right: six slots that can hold stacks up to 16 items each. One slot, however, is purple. Placing a potion's main ingredient inside that slot will give an AoE potion effect of your choosing. If it's beneficial, it will only affect players, tameable creatures, golems, and villagers. If it's harmful, then monsters will get those potion effects. All potion effects will only be at level 1, so keep that in mind. Customize your Abyslite to suit your style of play and expression!
Crafting Comes with a Price to Pay

Let's say you start harvesting Abysl from the Craftboxes, or even want to smash the Craftboxes for their experience. Well, the history of the Entrayls is a rather dark one, and meddling with Abysl will cause a particular Entrayl to be alerted to your presence. Entrayl's Venge is an effect that cannot be avoided, and trying to do so will bring death upon you. Trying to drink milk will immediately cause the effect's true nature to emerge, while dying will cause an explosive lightning strike, destroying any items that were dropped. You often have a while to prepare for the effect to end if you start harvesting Abysl, or a shorter time if you break the Craftbox.
Enough beating around the bush, we need to talk about the Builder's dirt- no, put that dirt block back, I'm speaking figuratively.
The Entrayl Sentinel

When the Builders fled the Nether, they left a few good men behind. One of them did not die in lava, and thus was sucked into the Soul Sands below him, trapping him. Calling and pleading, no one came, until an Orb of Dominance just so happened to float past and hear the poor soul out. Striking a deal, the Orb ripped his soul from the sands and brought him straight to where he wanted to be: with his fellow Builders in the End. Possessing one of the prototype Entrayls, he tracked down every last one of the life-crafting Builders who abandoned him. His soul cannot rest while the Orb of Dominance gives him undying qualities as per his wish, that is, until all Builders have been slain... oh hey wait, you're a Builder of sorts. Good enough justification to seek misguided revenge.
When the effect expires, the Sentinel will teleport to a position nearby the marked player and fly towards them. While the Sentinel exists in the world, all players will recieve bad luck, so taking care of him is vital to getting good loot again. The Orb of Dominance will float alongside the Sentinel, passively healing it and most of the monsters nearby, and even give them Absorption. The Orb can't be touched, but the Sentinel sure can.
The Sentinel offers three attacks. The first is a charge that can be blocked with a shield, stunning him for a brief period. This charge deals 6 hearts of damage on Normal difficulty with no Weakness applied (more fishing rod fun), so it'll either heavily damage you, or the shield. If you manage to block with the shield, be careful as it will still pierce through and deal half a heart of damage. Armor is ignored for the damage the player takes.
The other two attacks depend on the context: if there are no Endermites around, it will spawn a handful to draw your attention away. If there are Endermites nearby, it will spawn an Abyslite to cause further damage to you and your shield.
As the fight enters different phases, attacks will have shorter cooldowns, and things will get pretty scary pretty quickly. Keep your chin high and push through to send the Sentinel packing, leaving a fragment of his soul behind. The soul is used to create an Abysl Frame, and can even be used to levitate and slowly fall, making travel through the Nether and End easier, as long as you don't mind dealing less damage, and taking more damage... oh, and bad luck, that too.
Knowledge is Power

All of that talk about Entrayls, Abyslites, and death may seem overwhelming. If you really need help, you can trade with Librarians for collections, which will contain detailed documentation on Entrayls. They can even be traded for some potentially good loot, if you know the clientele.
Misc Features to Boot!

The End's got a bit more going on now thanks to the Entrayls' arrival. What will you do with these new creative opportunities, and a dangerous boss to boot? The choice is yours, Hero.
A bunch of advancements await those looking to master this mod, and as someone who enjoys the challenge they provide, I hope the same can be said for you.
Hope y'all dig the Entrayls mod! This was made in about two weeks to try making new things in Mcreator... yes I know, it ain't great, but I use UE4 and it feels very similar so its easier for me to grasp. I'd love to hear your thoughts on this take a late-game addition. Enjoy!
And no, I'm probably not going to bring this to other Minecraft versions... probably not. Most likely not.