Reminder of all the lore bombs Swain dropped on us
Most of you probably know that the demon Swain uses is able to reveal the secrets of other people to Swain. In-game Swain will comment on those secrets after receiving a soul fragment of the champion in question.
Here is a list of the most interesting quotes (and my guesses at what they hint at)
Edit: I've updated the list with info given from the comments.
Towards any Bilgewater champion:
They fight the mist above the seas, never looking beneath.
Is a great danger present below the sea? Maybe connected to the mist. Edit: Potentially referencing the swimming city from Pyke's lore or the darkness that changed Nautilus.
Towards any Freljordian:
They squabble as the winds howl, and the wards fade.
Further confirmation that the Watchers trapped in the Howling Abyss are coming back.
Towards any Ionian:
Magic spills from their land like blood from a wound.
Is Ionia losing it's magic? Edit: Maybe the 'wound' is meant to represent the weak separation of spirit and material world in Ionia. Perhaps a sign that the land itself is dying.
Towards any Shadow Isles inhabitant:
A king remains in the mist, and another marches.
The Ruined King is still present in the mist. The marching king could be referring to himself. Edit: I've been told the marching king is most likely Mordekaiser.
Towards 
She claimed it was the Frost Queen's grave. They believed her.
The origin of Ashe's bow is not what it seems. Edit: It's potentially of Targonian origin.
Towards 
What is this about... siblings?
Aurelion has siblings? Edit: Perhaps just a reference to Aurelion's lore when he was still in production.
Towards 
My my, does your dear grandniece know what 'C' has been up to?
Is Camille 'C'?
Towards 
The payment for her suffering in Shurima: a black rose.
Did Cassiopeia join the Black Rose?
Towards 
She's spurned so many suitors. A wonder they did not see why.
Maybe there's a deeper reason as to why Fiora is still single. Edit: Potentially lesbian.
Towards 
His secrets died with his sculptor.
Galio has secrets, but nobody knows them anymore.
Towards 
Ignorant of the very magic you carry into battle. How Demacian.
Garen has magic? Maybe he's unaware of it since he's wearing his petricite armor a lot.
Towards 
Faces fading in the flames. It was all her fault.
Jinx caused a traumatic event in her past? Perhaps the reason why she's so crazy.
Towards 
The first oath. The one she made to him.
Kalista has a special relationship to Hecarim or the Ruined King?
Towards 
He asked her to do what she could not. To kill me.
Katarina was tasked by Darkwill or her father to kill Swain.
Taunt towards 
Do they know who killed your father, or do they still blame me?
Pretty self explanatory, Swain didn't kill General Du Couteau.
Towards 
So many obsessed with the hero. Tell me of the hammer.
The origin of Poppy's hammer is might be interesting.
Towards 
She was closer than he realized.
Perhaps a reference to the events that caused Warwick to leave his criminal life behind. Perhaps related to Jinx or Vi.
Those are the most interesting quotes I found. If you're interested in the quotes I left out, you can find all of them here.
and is most likely not connected to the Mist.
, he can control the Mist at will and he is coming to Noxus to conquer the Immortal Bastion back for himself.
's lores and VOs inform us that Aurelion Sol's isn't the only cosmic dragon.
is the murderer her own father. Why? Because when a coup takes place, the people loyal to the old regime have to be eliminated.
idiot gave it a nice long look at it's mind, and another poor soul
was assimilated partially....and with her, all they needed to create fully functioning creatures....ones not merely shapes in energy, but living things.