Weird in-game message?

Hi, I was just playing a match here https://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/3209029686/42533202?tab=overview We took Baron towards the end of the match and started a teamfight and a big (probably 40 point font, gold) text popped up on the screen in all caps, and I forgot what it said exactly because I was surprised and confused. Something along the lines of "TOGETHER WE ARE ONE ENTITY--" or something ominous like that. It didn't show up in the replay, but the rest of my team said they saw the message appear too. It popped in like an important event or something. I'm wondering if we somehow triggered some weird easter egg in the game.

SOLVED: "Oh the eternity we shall spend together."

Okay, so apparently there's a quest with Thresh vs. Senna, kinda like Kha'Zix vs. Rengar. I've never seen one of these quests completed, so it just kind of popped up in my face and was weird.