He sent invites to everyone that he knew. The Fatui was hosting some kind of masquerade ball, and he wanted everyone to be there. Sure, it was an important event for the Harbingers.

But he also wanted his allies and friends to have a genuinely nice time. Maybe he knew, maybe he didn't. But his most recent ally was really just a spy.

Diluc had been invited, and they went in his stead. So when Childe saw them all dressed up instead, his grin widened.

”We have a surprise guest tonight..”