Sorry if it is a delay, but I think that it talks about the ruined king of the Shadow Isles. It makes sense.
I think that all of the songs talk about the king, and how the Isles' undead/creepy dudes army fight in a war, or about the way the once-trusted king gave up on everything and opened the gates of the underworld for the Shadow Isles (BTW, that's basically the story of how the Isles went from a peaceful land to a corrupted place where shadows and afterlife horrors lay).
I know that
Karthus,
Hecarim and
Mordekaiser were the ones who were really loyal to the king and followed his path of madness of allowing the undeath to flow into the land and "purify" it (Although, Mordekaiser is, as hinted in the female Twisted Treeline altar's quotes, a prisoner of the metal suit, and has no choice but serving the king).
There are other Shadow Isles characters, of course, that don't directly serve the "Ruined King". For example:
Elise is basically just worshiping Vilemaw, the Spider God, so I donn't think she has any direct interactions with the king and/or his kingdom.
Evelynn has no real story, yet, until she gets her relaunch/VU, so we currently don't know what's up with her.
I'm not sure about Thresh, but I think he's just a lonely and mad undead warden.
Yorick.. I think he's just a grave digger. Can't remember his lore, but I don't think the lore nor the Altars ever mentioned any facts that says that he works for the king. But he, like Karthus and Hecarim, embraces undeath very much.
Maokai is a spirit whose freedom had taken by magic, that turned him into the living, twisted treant he is. He doesn't really embraces undeath and just wills to rest and become a free spirit again. For now, he does the Summoners' bidding.
TL;DR: The songs of Pentakill probably talk about the Shadow Isles and the tale of the Ruined King (of the Shadow Isles).
P.S: I recently found out about a Rioter named Tom, through Twitter, who works in the lore team, that said that they have been discussing about the Ruined King.