Here's my little theory of the Shadow Isles and its people:
Long ago before the Rune Wars there existed two islands. One was led by a queen and the other was ruled by a rageful king. These are now the altar spirits. Both raged wars among each other, the queen using humans while the king used centaurs, which at the time were very widespread. The queen wanted the wars to stop however the king was bloodthirsty and wanted more, as he was starting to win these wars.
Hecarim, the centaur chief that was the right hand man of the king started to be more powerful and soon unbeatable. Desperate, the queen started to look for a way to end the war. She heard of an armor that protected the attacker from the most vicious of attacks and a mace made of the sturdiest metals and diamonds in Runeterra, however she only heard speculations of these weapons. Then, a messenger came with a message from the king: "Prepare yourself woman, my army shall destroy all and I shall conquer your land". Those words devastated the queen and so she searched until she found a possible map for the armor and mace and then called Mordekaiser. Mordekaiser was a very good soldier and magician, but he was a sadist and enjoyed the pain of others. The queen overlooked that and enrolled him in her army as he was very strong. She tasked him with finding the artifacts, and if not never to return. Mordekaiser accepted and went on his journey.
Eventually, the king found out and send Hecarim to kill Mordekaiser. However, soon Hecarim became Mordekaiser's ally in his journey. They found the cave which held the powerful weapons, there also was a girl who wanted them. She was Evelynn. She tried to touch the armor but she felt amazing pain in her body and a rapidly growing thirst for famine. She then got attacked by Mordekaiser and fled from the cave. As Mordekaiser and Hecarim got close to the 2 weapons, the king was almost at the queen's palace and was ready to destroy it. Mordekaiser then touched the armor and stated to wear it. As he wore it the armor fused to his body and his flesh got transformed to metal. The mace also came to him and he started absorbing pestilence around him. He also triggered a cataclysmic explosion which absorbed the life of the island.
The king and queen were killed and later transported in the altars on the Twisted Treeline. Hecarim died, but Mordekaiser revived him for mysterious purposes (this is why he is not to be trusted by the altar) and granted him powers. He stated to be called the Shadow of War. The island got deserted as the population died and the vegetation died too, only one tree survived which then got reborn. Tales circled about how the armor was a scheme by the Spider God witch has greater planes then we know.
That was my theory. Its pretty long, but I think its good. Feel free to read this when you are bored and then leave your opinion.