Shadow Isles is Reverse Shurima Theory
Ok, so before I start, this is just a theory. I know this is probably completely wrong, I just had an idea I thought was neat and wanted to share. That said, BRING ON THE DOWNVOTES!!!
So, as we all know (hopefully), Shurima was this great empire that, after Xerath intervened in Azir's ascencion ritual, was completely blown to dust, minus all the ascended (and however the hell Sivir got out). We also know that in the Shadow Isles, the king, seeking immortality (like Xerath and Azir in Shurima), great power, and almost godhood, ordered his mages to stop death, and caused a similar reaction, killing most everyone and everything (safe Maokai, and the original undead. Now, going off of what I know, there are VERY few ways to do this as a human. One way I know of is Ascencion, via the Sun Disc in Shurima. It would be nigh on immpossible for ordinary people to do this. So, what if, the two alters on the Twisted Treeline hold a power much like the Sun Disc, where certain rituals can activate their power and grant immortality and all that other stuff.
So, here is how I see the destruction of what the Shadow Isles used to be, went down. At first, the people like Mordekaiser and Hecarim (the people that would be like Nasus and Renekton) went through with the ritual and became who they were, but after a failed ritual (similar to how Azir's Ascencion went down), all the life was destroyed and/or killed, except for those who had already undergone the ritual.
This leaves a question though. If those who would create the ritual died, how are more undead growing in the Shadow Isles, and why are they not as strong? Well, I believe the answer to this would lie in Mordekaiser. As one of, if not the first, undead, he would've seen this ritual many times and would know how to re-enact it, hence him being able to bend the souls of the living. The reason why many of them are far weaker than he is though, would have to do with a reluctance to create those with as much, or more, power than he has, making him less dominant or because he cannot do the ritual with unwilling souls as well, thus resulting in weaker undead (hence why champions like Karth us and Thresh are as strong, or stronger than Mordekaiser).
Well, that's all I got. Sorry for the wall of text, and thank you for reading this far if you have. Leave your thoughts down below, if you can, as I'm very interested in what others think.