Imagine if you will a Demacian character who fights for good, but wields a dark power that was taken on by choice (and not forced upon them like Kai'sa, Yorick, etc). Vayne doesn't count as she doesn't use powers, just skill and her newfound touch of sadism.
How would you write such a character's story?
This is just a rough idea, but here it goes:
[Insert character name] is a member of the Dauntless Vanguard (Demacian Elite led by Garen, commanded by Jarvan IV) who was raised in a border village of Demacia where his parents secretly practiced dark magics.
After Jarvan IV's return to Demacia and recruiting Shyvana into his guard, Jarvan was determined to unite all peoples of Demacia, regardless of the Demacian leader's suspicion of all magic. [Insert character name] admired this sentiment, and justified his parents use of dark magics, beliveing that magic could be harnessed for good.
[Insert character name] participates in a battle against Noxus and witnesses Swain use the power of the Demon of the Immortal Bastion to slaughter a platoon of Demacian soldiers. Horrified by the carnage he decided he would find a power of equal or greater power, and would never be helpless to stop such a massacre again.
[Insert character name] learns of a vault the Demacian leaders commissioned to seal away all dangerous magical artifacts they encounter, hidden beneath the pedestal of the Colossus Galio, where no sorcerer could ever get to it. Being Demacian military and having no innate magic of his own he snuck (sneaked?) into the vaults, and incapacitated the guardsmen. He perused the halls of lost treasures until a [Insert weapon type, Sword, Axe, Bow, etc.) caught his eye, mesmerized by its call, he heard a voice. His own voice. It called out to him and promised that were he to abide by its rule, he would never again be powerless to defend his comrades.
And thus I give you the last Darkin, a Demacian with a just heart, a Dark(in) and willing secret, and I finally get my last Darkin.
Thoughts?
~ Sparky 
Cecil, the protagonist of Final Fantasy 4 is a good example (The original Dark Knight), although he started as a Dark Knight and later became Paladin
Maybe character is a spy in evil side. And has some interest in their power in scientifc approach. He needs (and wants) to use this power to stay unsuspected, meanwhile providing information about evil plans for Demacia.
A prince (J4's sibling I guess), who was a knight in training left with his men to attend the pressing matter of corruption from either the Void or Shadow Isles, probably the Shadow Isles.
Through the horrors he'd endured after arriving at Shadow Isles, the prince eventually lost his stand in his conquest and began to seek forbidden and sinister means of bolstering his power to win the battle, which his desperation eventually led him to a cursed sword.
Despite his right-hand man's warning not to, the prince took up the sword. At the moment of truth, the right-hand man realised that the prince had since gone mad, as the prince struck his loyal man down. The prince was no longer the same prince Demacia knew of.
Unbeknownst to the prince, it was all plotted from a long time ago by the unseen will of a powerful being, who had been interested in acquiring the prince into their ranks, for the prince to lose sanity to eventually taking up the sword and become their servant.
With the command of the being, the prince then returned from the Shadow Isles, under the name of a victor who had conquered the Shadow Isles. The city welcomed their beloved prince by rolling out the red carpet, showering the pathway with flying pedals, and filling the air with the ever-exciting fanfare. The king, Jarvan the 3rd, welcomed his accomplished son with open arms, as the prince arrived at the throne room and kneeling before the king.
The king asked the prince to approach him, it was a proud day for him, both as a king, and as a father.
Then suddenly, the prince stood up and violently approached the throne. He viciously grabbed the king on his shoulder, with his sword pointing at the old king's neck.
"What is this? What are you doing, my son?" The king felt nothing but confusion. This turn of event completely flabbergasted the festive king.
"Succeeding you, father..." The cursed sword stabs through the king's throat, as his crown fell and cracked on the floor.
The prince was no more, he was a Dark Knight.
Honestly, if you make 'guy becomes corrupted by demons or whatever and become a "Dark Knight"' story and have them be from Demacia, I think it's inevitable that the first thought most people would have is to compare him with Arthus.
I think a better twist on this trope would be if a Demacian high ranking officer defects from Demacia for some reasons and go to say, Noxus, and learn to use magic for combat, something that is generally forbidden in the lands of Demacia.
He'd be questioned by Noxian all the time, and be cursed by Demacians, which slowly drives him into depravity as he tries harder and harder to cut ties with his past affiliation and and earning his new faction's trust, only to be met with more distrust from Noxus and more damnation from Demacia.
But idk.
I'm still not clear on the distinction between 'dark magic' and any other kind of magic.
How about a Demacian Socialite that rivals Jayce in popularity, prestige and wealth?
Or maybe a demacian night gone corrupt but still accepted by them, or at least some secret government guys.
Give them a Darkin who claims to be the Veiled Lady. Either from corruption of the mind or religious extremism the champion believes her.
First thing's first, from what point of view are we viewing this power as dark? If it's the point of view of Demacia then Lux already fits the bill. She wields a power her country views as wholly evil after all, and wields that power for the benefit of Demacia. To put it simply, from Demacia's point of view Lux might be two parents shy of becoming an ultra peppy Batman.
If we take a non-ridiculous point of view I guess it would depend on the type of power being wielded. Magic on Runeterra seems to carry the intent of the caster, rather than being something with innate good and evil magic, so what is done with it matters far more than the magic's source. It would have to be used by someone that can effectively work in the shadows, and would have avoided characters such as Vayne. I could see something like an advisor to the king, or possibly the king himself, wielding the very magic that the people fear, in an attempt to keep them safe. Someone no one would suspect, and someone who is in a position to silence any who might find out.
