Mordekaiser Lore - The Theory
Alright, let's go over what the lore update mentions.
"All things must die... and yet I live on."
The baleful revenant Mordekaiser is among the most terrifying and hateful spirits haunting the Shadow Isles. He has existed for countless centuries, shielded from true death by necromantic sorcery and the force of his own dark will. Those who dare face Mordekaiser in battle risk a horrific curse: he enslaves his victims' souls to become instruments of destruction.
Now, let's go over all of this: Mordekaiser is now being more connected to the Shadow Isles than before, which hasn't been the case since his first lore summary, with the ones following simply mentioning him lurking around Noxus like a forum troll, and the other just saying that he predates the Shadow Isles, and the most recent just saying he is on a mission. This is interesting, because a good majority of the Shadow Isles champions are mysterious, but a lot is known about them, either through the context of their story or through their actions.
Kalista is mysterious but has a defined role that everyone can understand, and Karthus is pretty basic as his story is pretty much told already. Hecarim and Thresh are mysterious but their behavior is enough to give an idea of who they are, with Hecarim loyally acting in the name of the Shadow Isles, and Thresh simply being a complete dick who torments the souls of the deceased and pissed off Lucian. Elise is pretty straightforward with her association with Vilemaw and being intimately tied with the organization of The Black Rose, while Yorick is a spectre of the isles intimately connected with the passing of the dead through the isles itself, and Maokai is an unwilling product of that which turned the isles into what it already is.
Two champions, however, are shrouded in complete and total mystery: Evelynn and Mordekaiser. And there simply isn't enough to know about Evelynn to suggest anything, other than a rivalry with Twisted Fate, which could mean anything considering Fate's own nature. Mordekaiser, on the other hand, is mysterious because despite there being so much to his description, there is so little to know about who he is or what his motivations are. He is connected to the Shadow Isles, through friendships with loyal Shadow Isles denizens, yet is always hinted that the isles are not his priority, and that he is seeking something, and is willing to do anything and seek out anyone - willing or not - to find whatever is it he seeks.
However, there are things we can deduce about Mordekaiser: he is an undead who is specifically known to feel pain and spread his pain and suffering to others, either directly or through some form of pestilence or disease. He is tied intimately to his armor, which is often suggested to serve as a vessel and prison for his form, suggesting a level of unhappiness with his situation, further emphasized by two things: his status as a revanant, and being a malicious and hateful spirit. Revanants are undead who survive and cannot find peace until they fulfill some sort of mission or obligation or duty, and the mention of him being hateful indicates that he is unhappy with his situation as a whole. By itself, it doesn't tell much, but a lot can be figured out with this, in addition to the new lore summary that was added just recently. It may suggest that Mordekaiser died unwillingly, either through being killed or through some other means, but his willpower and the nature of the necromantic magic that sustains him kept him alive... in a sort of way, and Mordekaiser ain't happy about his situation, which indicates he does not want to be an undead.
That is one of the biggest things that can suggest something about Mordekaiser's origins, if you can sum everything up together.
First and foremost, Kalista and Mordekaiser, both are not what you might call happy undead, but there has to be a reason they are unhappy while the other spectres are either happy with their situations or making the best of them. With Kalista, it makes sense: as the spirit of vengeance and retribution, she deals with unhappiness in all of her dealings with other souls, and it can be assumed that she was betrayed and thus took this form to attain retribution, and now she fulfills a higher role as a manifestation of a concept, hence why she is not considered an enemy by Kindred. However, it is obvious that Kalista is no longer just herself, so her motivations are no longer singular and her personality is now a sort of collective of everyone who was betrayed and made a deal with her for revenge.
That in itself says something: that Mordekaiser and Kalista are not associated could indicate that it is not revenge that Mordekaiser seeks, which one would think is the case of many revenants. The fact is, if revenge had been a motive at any point, then surely Mordekaiser and Kalista would have met at some point, which might be the case. But, if they hadn't met, and Mordekaiser was not out for revenge... then what could he still remain to do? What could he seek to do that has taken countless centuries, and he could not do by simply focusing on supporting and expanding the Shadow Isles, if not possibly leading them? If he wanted to rule the world, he could do so using the Shadow Isles as a platform to mount an empire befitting a 'Kaiser,' but his endless quest suggests that the death and violence and suffering he spreads is a purpose, a means to an end, and not the objective itself.
What if, perhaps, the end game is not death, but life?
What if Mordekaiser wants to become living once more? And not a mockery of life like the undead, or some abomination of such like Sion, Urgot, or arguably (who is more machine than human at this point) Viktor? Instead, he may wish to return to a mortal form, be free of undeath and being bound to his armor, and live the life that he always wanted. Perhaps he seeks to revive more than himself, maybe others precious to him in his human life, he could have had a family, parents and siblings he lost, or a lover, a wife and children perhaps? It could be easily explained that he became a revenant because he simply refused to die througna combination of his willpower and necromantic powers - which could have been researched for his goals - and now seeks to return to life once more?
Such is not a mission that can be achieved in a small amount of time or through mere vengeance, thus meeting Kalista would serve no purpose to him, and it is not something that the many members of the Shadow Isles are focused on (Karthus, Hecarim, Yorick, Thresh, to name a few). However, it would explain a few of his actions, his endless and seemingly inexplicable quest, his seeking of information (the tale of him interrogating the master of a young girl), his association with the Shadow Isles (in case one denizen was, like him, trying to reverse undeath, which is very, very likely), even - prior to the retcon - joining the League itself could be explained, as using the League to find the answer to regaining life (something the League might have been interested in and even unofficially supported his efforts in, since who wouldn't want to know how to come back after dying?).
Even the pain that he experiences may be an indicator, because pain is - above all else - something the living feel, something that helps to prevent them from actions that would inflict further harm to themselves, and helps to remind them that they are conscious and alive. Mordekaiser being able to feel and spread pain, may indicate that he is trying to hold onto that part of his humanity, because what else could make him feel alive in his state of undeath? His necromancy is an obvious nod in trying to learn to revive the dead, a sign of his ambition and possibly an outlet for his anger against the living, who do not appreciate what they have in life, and spreading pain may be 'teaching a lesson' to the living that pain is as much a blessing as it is a curse.
It's just a theory, but one I think fits a man with ambition and determination like our Iron Revenant.
