The Shadow Isles is not touched upon like other places because Riot wants to keep everything a mystery for now. But here's my idea.
There was once a king who tried to bridge the gap between life and death so he could become its master. It worked, making everyone in the Shadow Isles undead if they survived the bridging. Unable to die for good, they had no purpose but to go to those with strong wills.
and
command hordes of specters and undead.
simply became the eternal grave digger before the Shadow Isles fell, and he didn't really care since he's living in his own torment.
is a creature of sorts, more of a demon than a lich or undead, and her purpose is unclear. She might just have this lust for hatred and pain. And what better place to find the two than the resting places of many souls?
Karthus is an interesting one. He was born amongst the living, but his desires and interests were with the mysteries of death. He heard of the Shadow Isles, a place where the bridge between life and death was closed. Without hesitation, he traveled from the living to the realm of the undead, becoming a lich with his knowledge of death and the arcane arts. His power was limited, he needed souls to fuel his power. Thus, he used his articulate speech and soothing voice to preach to the souls of the Shadow Isles to join him. He also preached to those lost in life, urging them that life was meaningless and death was a horizon for greater purpose. Soon he grew in power, becoming the Karthus we know today. However, he started enticing the idea of having followers to sing in unison with his Requiem. Thus, he continued preaching death to all, and even now he hears countless voices in unison.
is pretty self-explanatory. She was betrayed by someone in the Isles' court, perhaps the king himself. She might have been an advisor, a commander, or even a queen with battle prowess. Regardless, when she died, her soul became separated before the bridging, and thus her soul was simply infused with her armor. Without much else purpose and a burning hatred within her, she honors the cause of vengeance upon traitors, heeding the call of those who have been stabbed in the back.
is one who doesn't need much explanation. She willing gave herself to Vilemaw, becoming his eternal servant in exchange for eternal youth and life. She also gained immense power from this exchange, and she uses her beauty to trap unwilling devotees to the spider within Vilemaw's web. She is a temptress and a charlatan, and is very separated from the rest of the Shadow Isles. In general, I suppose she came after the bridging.
I'm done. Feel free to tear my theory apart.