What if a necromancer brought back Jarvan the first in the shadow isles and he was the "Ruined King"
The Ruined King in this context being the body of the old one, basically fusing personality wise with Jarvan the first
Taking you seriously here:
There are some problems about it though.The problem is that the shadow isles are already a thing around the time when sion was created.Boram became paranoid after sions death and seeked the necromantic knowledge of the shadow isles to bring him back(which the black rose did,just not the way boram imagined it).
For J1 being the ruined king he must have been resurrected almost instantly after his death and then quickly made the shadow isles a thing. Then there is the fact that it was said by kalista that the ruined king was overtaken by grief,which caused him to create the shadow isles,but if j1 was an undead without much emotion,then how was he overcome by grief?
Possibly using his body as a vessel for the Ruined King, to augment the power of the Ruined King possibly because the original body was destroyed. Imagine the power increase of the Ruined King being fused with the legendary first Jarvan. Scary.
The vessel for the King being destroyed and them not knowing what to do with it until they found a vessel could somewhat explain why
exists as well. Possibly the first attempt? Possibly the Ruined King's version of
?
So its literally Jarvan the First but not actually Jarvan the First. Personality could become fused in some way, which would make an interesting character, but probably not.
http://goodwillwatching.com/wp-content/uploads/2014/11/Keanu-Reeves-in-Bill-and-001.jpg
First made it as a joke but then I realized how legit this could potentially be. Just think of it like its a plot line from something like Game of Thrones.
A great warrior kills a great king. The great warrior is brought back as a more terrifying version of himself but without morality or humanity thus making him at least 5 times stronger and more influential than before. What would happen if that great king was brought back? What would happen if that king had no morality or humanity anymore and could achieve far more than he could ever achieve and fight over 5 times better than he ever could have while he was alive?
Also imagine how stressful life becomes for 
He has to run a country that is warring with
that summons back the man who killed his great great grandfather and who also started the war with Noxus with the help of
posing as him
Now he has to deal with his father being returned as just as much of a demon as
or potentially some ancient demon using his Great Grandfather's corpse as his own new body.
I personally prefer the theory that the Ruined King was in fact once the Rune King, and ruled the magelords before their destruction at the hands of the Protectorate.
What if the Magelords weren't entirely destroyed like you say, and their children eventually became Demacians? The Protectorate could have became the Noxians which would be fitting to League's Lore narrative in the way that the Noxians and Demacians have always hated eachother, and maybe they have literally always hated eachother as long as time has existed?
So in order for the Ruined King to have a proper vessel, since he might have been destroyed, he would need a relevant body with the same bloodline of his which would explain Jarvan the First being a potential vessel.
This also gets me thinking, what if the Rune King was actually a Darkin just like
and thats why he chose the Protectorate instead of just picking the losing side? He would have a real motive for that result rather than just enjoying bloodshed.

He could potentially start an entire undead army and throw the Shadow Isles into chaos thus causing what
's lore alludes to.
Was it
that summoned the King of Demacia back from the dead?
Possibly could have been
or even
by mere accident
does % of target's Current Health as Physical Damage and is said to be a fraction of the Ruined King's real power
's passive is "Martial Cadence" and is no doubt a passed on technique from his forefathers. HIs passive is you dont already know is % target's Current Health as Physical Damage every 10/8/6 seconds based on level
We know its probably a passed down technique because
taught it to
and the active of
is practiced in some way by 