Have you ever felt the light?
Its shimmering light, showering you with comfort and warmth. It feels good, doesn't it? To bathe in it for as long as you could ever wish to, you would be willing to do everything it tells you - every command, every order and every word it whispers - you hang onto it as if it's the air that your lungs survive upon. Like the earth beneath your feet that keeps you from falling. Like the blood in your body that keeps your heart beating... and alive.
I reckon that you have never felt its burn.
It is easy to forget... or should I better say, its easy to dismiss it... the way it punishes you, only to lure you back in again... the way it blinds your eyes, although you continue to let your eyes remain open to its luminescence... the way it stings you, yet again bathe you in the light you so love...
You will strive... harder and harder to make sure you are never punished again. But that's not going to be enough... your best will never be enough... when you feel like you have finally accomplished the impossible, it will tear you down... scorn your stupidity... reprimand you for taking so long on such a simple task.
But like a demonic angel... it will comfort you, telling you to do another task to prove that you are worthy of its sanctuary. And this process will toil on, day after day, year after year. The same endless cycle...
It will tell you to get better... to keep getting better. And you will, to the best of your ability, pushing every limit within your body for the sake of its warmth and love. For every ounce of sweat your body drips, you believe you will be rewarded just as much more...
Eventually though... just like that one day, as I stood in wait for it... it will disappear from you. Vanish without a trace. Not even a single word of farewell or reassurance... as if it never cared that you existed, although you needed it most.
At such a time...
What would you do?
I live my life now, in my own city, in my own realm, in my own Domain.
No longer will I tolerate such cruelty. The light is nothing more than a trap... a temptress, luring you in with an addictive honey to only capture you in its shackles. And as long as you are in those shackles, it will tell you sweet words that deceive you at every corner. Even if you know there is no truth in what it tells you, unfounded hope keeps you from doubting them in the end. Even though it gives nothing in return for your efforts that you have given. Those sweet words never promised a single reward in the first place, yet you are attracted by the possible surprise it may offer. That it will at the least say to you, "well done."
But... it pays no respect to you, treating you nothing more than a hollow existence. A puppet to its fancy show...
Thus... I no longer care for it...
So, hear me now, Agents of the light.
You are not welcomed to my kingdom.
Any light that dares try to illuminate my realm will be snuffed out by not only me, but by also the darkness its dares expel.
You may believe that the Darkness... is cruel or scary. But I inform you, it is neither cruel or scary. While it may tell you the cold hard truth, it does that so that you are in mindful of the situation. And it is up to you on how to react to this newfound knowledge.
And I will tell you: I am not evil.
I do not seek the servitude of others under me, nor do I revel in their pain and suffering.
I am not chaos.
I do not desire to shake the order of this world out of balance, nor do I wish to cause a ruckus in every corner of this universe.
Then what am I?
I am Darkness Incarnate.
I do not hold the innate impulse to kill anyone. But... I will if the light dares stay after I have made my warning. I do only wish to live peacefully on my own, but I also welcome those who no longer wants to associate themselves to the light.
I am Darkness.
My World... My City... My Realm... they embody the black depths, for it is a sanctuary. A sanctuary away from the sun that forcefully hails in the sky during the day. A sanctuary where the moon no longer guides lost travelers with its falsehood. We wish to have nothing do with these artifacts.
I... do not bring anybody down. I offer all the pleasures the light has to offer without dragging along its cruelty. When somebody is hurt, I cure them. When somebody has fallen, I help them off the ground. When somebody feels oppressed, I will spread my arms, welcoming them into the embrace of my community.
But even with such kindness, I will not tolerate anything associated with the light. Those who wish to cause a rebellion, something the light created to cause a discord within the darkness, I will immediately purge you from my city. Those who wish to start religion, something the light created to control the minds of commoners with false truth, I will be banish you out of my realm. And for those who wish to spread the light... will be given the greatest of punishment I can offer for their crimes.
The Penalty Game.
I wish for the people who embrace the same darkness I cherish to grow just like I did, but without having to go through all the trials and tribulations... all the pain and suffering... all the wrong turns I had made to reach where I am now. I wish for them to be feel empowered... be capable... think for themselves so that they can face every adversary without losing their innocence. If they have found the same salvation I had found in the darkness, I will protect them against the light and spare them all deceit it is swollen with.
Now, who am I again?
I am the Protector of the Ebony. The Herald of the Black Crown. The Harbinger of the Shadows.
I am Darkness Incarnate.
I like the core idea of shifting forms based on specific domains. However, my number one complaint is that your kit is just too wordy. It took me some time to read through the whole thing, and more time to understand exactly what was going on, especially since there were several buffs and debuffs that applied to multiple abilities but weren't phrased too clearly. Without even needing to take anything away from the kit, you can phrase everything more efficiently. For the passive, for example, I'd take out the details and instead create a separate section for Crystals of Darkness, with the passive description only painting the broad lines of what they do (e.g. "Erebus has his own trinket that creates special wards, which transport any enemy who gets too close to his domain").
My other big criticism is that the kit looks like it lacks direction, and some components don't really feel like they'd be intuitive in an actual in-game scenario (i.e. disarming oneself to reset an ally's autoattack timer, which is very mechanical and not thematic at all). Some effects, like Beloved Ward and Devourer of Light, both have great core themes, and I think you should focus on making Erebus a vision-manipulating support. Here's more detailed feedback for each ability:
Domain of Erebus needs to be streamlined. The core idea is brilliant, but nobody reading your kit at first glance is going to care about how your champion's special wards' health works. Since these Crystals of Darkness are objects, I'd make a separate section describing their details, and would reserve the passive description for the barest essentials.
Stygian Prosperity doesn't look interesting. I don't think resetting any autoattack timer, let alone an ally's, should ever be the focus of an ability. This ability is basically a more complicated Sheen proc for an ally, and I think there are many more interesting things that can be done here.
I like Beloved Ward connecting Erebus to his ally, but again, on-hit damage just feels meh. Perhaps you could capitalize on the movement speed when moving towards each other and make it a constant buff. I also think mentioning Nefarious Warden here feels weird, since the effect is described in a separate section and is a complicated debuff, and I personally don't quite see what it's meant to do here.
Devourer of Light is a great ability. I'd streamline it a little bit by making the zone a proper Erebus's Domain and making it stick to the target, but the core idea is quite good.
Blade of Dusk's wording is a bit weird. Is it just one attack, or does it last for a duration? I'm also not sure what it's meant to do in the grander scheme of things: is Erebus meant to turn into a fighter when inside his domain?
Imprisoning Charge looks good, though I think you could again simplify the effect down to preventing Prisoners of the Dark from leaving Erebus's Domain by any means. Also, if the effect deals damage, you might want to indicate it.
Nefarious Warden looks like a bunch of different effects strung up together, and doesn't really seem all that cohesive to me: what's it meant to achieve? What's the intended gameplay? Having it as a bonus you can activate from multiple sources makes it seem like it's an important part of Erebus's kit, but I don't really see what kind of impact it's intended to make.
"Penalty Game" doesn't sound like an appropriate title, and sounds like something out of soccer/football. The effect itself looks really fiddly, and the payoff is basically three different flavors of bonus damage, which I don't think is worth it. I'd look for something else, if there needs to be an ultimate for when Erebus is inside his domain.
Overall, you have a great core concept, but also a lot of work to do to polish your kit, and build it into something with more cohesion and interesting gameplay hooks. I'm here to help, and whatever I can do to contribute, whether it's give feedback, suggestions, etc., don't hesitate to ask.