cersei mentioned it a few episodes back, the one where sansa and tyrion got married - margaery called her ‘sister’ and cersei was like BACK THE FUCK UP BITCH DO YOU EVEN KNOW WHAT HOUSE I AM FROM

and she told the story of house reyne of castamere - they were lannister bannermen who rebelled against tytos (tywin’s father). tywin was like I DON’T THINK SO I AM A GOD DAMN LION AND CHARLES MOTHERFUCKING DANCE and he fucked shit up so hardcore like he marched with lannister forces and literally murdered every single man woman and child of house reyne

the song ‘rains of castamere’ is about what a stonecold hard fucker tywin lannister is and how you don’t fuck with him or you and everything you love will die - now the rains weep o’er his hall, and not a soul to hear

it’s been used before on the show, and in the books, by lannisters or lannister bannermen (lannermen) as an explicit audio clue to express to someone ‘don’t fuck with us or your house will get the same.’

cat heard the tune and - knowing that it is an unbelievably inappropriate song to play at a wedding, as well as its context of lannister aggression and dominance - knew that shit was going to go down and it was going to go down hard