Ayy let's theorycraft a skillset, guys!
(Giant wall of text INCOMING!)
So what should his passive be? He has an obsession with death, but he himself should be largely unaffected by it. Someone mentioned an obsession with reviving his Queen... so maybe something like this?
I'm gonna be going in two different directions - AS/lifesteal tank/bruiser (the skills that start with "King's") and support-ish lifesteal tank (the one-word abilities).
Passive: King's Redemption
Upon death, the Ruined King latches his soul onto the nearest enemy Champion, siphoning a % of their max health in order to heal nearby allies for the same/reduced amount. (Maybe make it scale with his own Health as well?)
If he's a Tank, this could work. It would tie in with his obsession with life and death and his penchant for delving into unknown magics, perhaps sacrificing even himself, to protect those he loves.
Passive: Allegiance
The Ruined King can Bond with an allied Champion. He shares the Bonded Champion's pain, reducing the damage they take by x % - this damage is redirected to the Ruined King himself. This effect is nullified when x units away from the Bonded Champion. (If his Bonded ally dies, if they both accept, the Ruined King can sacrifice his own life to revive his Bonded ally - very long CD).
This one could be really interesting should he be played as a Support. Can you imagine the lane interactions with Kalista?
The add-on in (brackets) is tentative. I'm not sure if it's a good idea to add, since it could be potentially broken (idk in what ways, but it's always a risk with revive skills).
Q: King's Smite
The Ruined King imbues his blade with power. His next attack is converted to AoE (maybe a % of his damage instead of total, scaling up to 100% or more at max rank) applying on hit effects etc.
If used after having casted King's Command, the damage is dealt instantly around the sword (and himself, maybe?) instead of next attack.
It's simple but effective. Works well with both BotRK and Hydra, the AoE can apply both lifesteal effects (plus the % HP from BoRK in an aoe!) May need some toning down to be balanced (since Shyv has it on her Ult empowered Q). I also like it's interaction with his E - throw out sword, and poke.
Q: Ruin
The Ruined King strikes a vicious blow (a bit longer than Kha's Q range, probably) to target enemy Champion, dealing physical/magic damage & afflicting them with a debuff for x seconds. Any subsequent spells and/or attacks on the debuffed Champion will heal the Ruined King/allies/Bonded ally (not sure if it should be selfish or team-oriented) per hit/spell.
Basically an offensive version of Galio's shield. Idk if it should heal allies, or just him (and his Bonded friend), though. Probably include allies, or if not, at least his Bonded ally, since he's support-ish in this case.
W: King's Regalia
Passive: The Ruined King gains x% bonus Armor/MR - OR - Lifesteal the lower his HP is.
Active: Increases attack speed by x% and granting him a shield that lasts x seconds. Duration can be increased (up to y seconds) and shield can be replenished, both by Lifestealing.
In tune with his a lifesteal tank mentality. Becomes deceptively tanky when low health, and can replenish his own shield by lifestealing. As long as the Armor/MR isn't too high, this could be a pretty cool spell.
W: Lament
Passive: Increases own attack speed by a small amount.
Active: The Ruined King sacrifices a % of his own health to heal target/Bonded ally, while also doubling the passive attack speed's effect and giving it to them.
Honestly I'm starting to stretch here, but anyway. Regretting his Queen's death, wants to help others, must satisfy cursed sword, yadda yadda. Y'all know the drill. Wasn't sure between a heal and an attack speed buff, so I just threw in both together xD
E: King's Command
The Ruined King throws his blade, which lands in target area after a delay, dealing more physical/magic damage and slowing those in the center, with less damage and slow to those closer to the edge. The sword stays there for x seconds, during which he can use Q. He can recast E once to - reposition it, dealing x amount of damage in the line it travels, and Q can be cast while it moves - OR - teleport/dash to the sword, refreshing Q's cooldown.
Not too sure what to do with this one. I like the interaction with Q, but otherwise it's just like Xerath's W.
I added an extra effect, but idk which one to use - move the sword itself + extra damage (like Orianna Q) or teleport/move to the sword (I prefer teleport over move, cuz Azir and J4 already have the latter - plus it should have a long animation and CD anyway, and it does no damage, so it can't be abused for mobility).
Between the two, I'm more for the self-displacement rather than moving the sword (easier initiation). But idk.
E: Subjugation
The Ruined King dashes in target direction, dealing physical/magic damage in a line, stopping at and slightly knocking up/back/stun/snare/slow/whatever the first Champion hit, draining its health (maybe a % of health, maybe a flat amount + stat scaling, idk)
Any self respecting tank or support needs CC. Not sure about what kind, though.
Okay, I'm probably gonna need some help for the ult(s). It's getting late and I'm tired, so my ideas may be getting a bit crazy. Bear with me xD
R: King's Trial
Winding up, the Ruined King throws (something cool, idk, a berserk lance, or something) in a line (around Varus Q range). If it collides with a Champion, the Ruined King and the target are both moved towards each other, meeting around halfway point. They are both Taunted to each other for x seconds, with the Ruined King taking reduced damage from his opponent. Both Champions cannot be stopped by any form of CC. The Ruined King can cast W during the fight.
This is a bit weird, but essentially he forces the first Champion he hits to fight him in a 1v1, and they both can't be stopped until the full duration is up. HOWEVER, they can be damaged normally, and thus be killed. It can also be bodyblocked by tanks. Should it be cast over a wall, they'll both land on the side the Ruined King was on. The windup is similar to Ez or Lux'z ults in that it's a casting animation that can't be interrupted.
R: Defiance (other potential names: Ultimatum, Provocation, Challenge, Agony, Crucifixion, Devotion. I'm really indecisive)
The Ruined King designates an area (pretty long range, but not global). After a delay, he disappears, then reappears at the area, taunting nearby enemies for x seconds. During the taunt, he gains x bonus (Armor/MR - OR - Lifesteal - OR - Attack Speed - OR - Damage Reduction - OR - A combination of the aforementioned things).
For the ult, either for the brusier or the tank, I wanted to make it some sort of forced engage. But in the tank's case, it had to be something self-sacrificial, so I made it a forced taunt.
Honestly, both spells could be interchangeable (the challenge on the support, the aoe tp/taunt on the bruiser) but I didn't want a high-damage bruiser to also be teleporting around the map xD Besides, it fits a support-ish style tank better.
K I'm tired, idk what I've done, I'm not proofreading and I'm not looking at if the concepts are broken. I really like the Champ's concept, though, so if possible we can just discuss potential kits. I'm all ears for changes!
"I was a fool. Now, I don't know what I am."
"You told me, 'no regrets'."
"You know nothing of loss."
"The battle has ended, but we are not yet free."
AGAINST ANY CHAMPION WIELDING 
"I would like that back now."
!
, one who embodies death itself. He would then get his own event when the other two Darkins are released and Kayle and Morg get new lore that makes them the Darkins of light and dark.
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