User blog comment:ND999/Lore Hypothesis, Mt. Targon, Guardian Sea and Shadow Isles/@comment-6582248-20141116014213

By now, we know that Mordekaiser, Kalista and Hecarim had some relationship, Hecarim betrayed someone, the three of them were there when all hell broke loose in the Shadow Isles, the King of Shadow Isles ordered his mages to break the line between life and death and used the life in the Isles to fuel the spell (Maokai lore), Mordekaiser is prisioner of his armor (which suggests something gone horribly wrong), Kalista was close to the Ruined King, Hecarim does not remember himself before the catastrophe and the altars say that he must remember and not to trust Mordekaiser, we must also count the existence of the Spider God (Vilemaw)

So... I can imagine that a Rakkor warrior named Kalista is recruited by the Ruined King (who i believe is Mordekaiser, the altars refer to him as "My Lord"), the ritual is to break the line between life and death and Kalista says that the Ruined King's reason was clouded by grief, so it could be a ritual of resurrection or a kind of "ascension" to gain control over life and death and Hecarim disagrees with altering the natural cycle and disrupts the ritual, causing undeath to spread all through the Isles, creating the Twisted Treant and Treeline, Vilemaw could be one of the source of the curse or a heavily mutated spider (like what happened with Rammus) the disruption also caused Mordekaiser beign trapped in his armor and Kalista, beign the King's guard, fought Hecarim and was killed by him.

Now that Heca does not remember that he rebelled against Morde and Kalista, he became the general of the army of the Shadow Isles, Morde seeks the way of gaining freedom (beacause the armor is like old Xerath sarcophagus, it aprisions his power and protect us all) and Kalista seeks revenge to every traitor