"Thornmail" was actually about the Poro King, not the Ruined King

Lyric analysis as follows:

My King! My King! How was I supposed to know that everyone will falter when you die?

Naturally "My king" is the poro king. The second half of everyone faltering was because of losing the poro king buff that would keep them alive, meaning that the people would perish instead of pushing the line.

And as we speak the army of our enemies Is approaching our gates, we cannot fly.

Weird lyrics for the ruined king right? Well, not if it's the poro king. Approaching the gates is the term for the inhibitor...BUT the "cannot fly" part is due to the second skill, To the King, which would let you fly to the poro king across the map.

Behold, my son, There's a way to save you all, Don't you despair, don't be petrified There is an armor, ancient magic made it strong, And you shall wear it when you face the fight.

My guess is that "Son" is Braum, probably the chosen of the Poro king due to his strong ties to the poro. This goes doubly true for the second lyric where it talks about him using the thornmail, as Braum is a tanky initiator.

Well done, my son, it's the way to save you all, but don't you think the realm's been purified. Creatures of darkness are still longing for your fall, Keep them in thrall, it's time to smite and ignite.

At this point Braum has summoned the poro king, the massive fluffy floof thing whispering in his ear. Braum shouldn't rest because the enemy team's "creatures of darkness" (AKA: Shadow Isle Poros that you can get) long for his fall. Braum is legendary and the team wants his bounty gold. However this is exactly what Braum wants. By "keep them in thrall", he is drawing aggro, thus meaning that they focus the tank rather than the poro king or the squishy assassins the rest of the DAMN TEAM PICKED FOR FUCKS SAKE WE DON'T NEED KAT YI AND TALON.

Anyways there you have it. Thornmail is about the Poro King game mode.