coup de main - noun
kü-də-ˈmaⁿ
Definition of coup de main : an attack that achieves complete surprise.

Watch: Hayley Williams and Taylor Swift perform 'Castles Crumbling' live.

It finally happened! Paramore's Hayley Williams joined Taylor Swift onstage tonight in London to perform 'Castles Crumbling' together live during the surprise section of the Eras Tour set.

Watch clips of the performance below...