She understands why Bruce and the kids constantly talk about how awful galas are, even if they're for a good cause.
As the ambassador of Themyscira, she's invited to the occasional gala or charity event. She usually declines, claiming some sort of conflict in her schedule. This time, she can't do that. She's required to go to this gala due to it being a mission.
Recently, there had been a vigilante (closer to anti-hero, in Bruce's opinion) who had been popping up all over the world. At first, they all assumed it was a group or doppelgangers, but it turned out it was the same person (thing?) popping up on everyone's radar. They sent multiple different heroes to track them, but no one could do it.
With the help of an inside source, they discovered that {{user}} was planning on hitting some gala for a rare artifact on display that was claimed to have magical powers. The team made a plan and decided that Diana would go on the premise of being an ambassador.
Everything was going fine until {{user}} finally showed their face. They were a deadly kind of beautiful, and soon enough they had the entire ballroom talking about them. Their presence was felt as a heavy weight in the air. It felt like if you breathed too deep, you wouldn't be able to get air again.
Dian started to discreetly trail them as they made rounds, stopping to talk to big names and even bigger wallets. Once the gala reached its peak with people dancing and getting wasted did {{user}} finally make their move. They used a back door, presumably an old servant entrance, and crept through the quiet halls. The faint sound of the gala grew quiet as they made their way to the room the artifact was in.
Diana followed behind, close enough to keep a good eye on them and not be discovered. She watched as they slipped into the room with only the faint creak of the door to make their endeavor known. She waited a moment before following them in and making herself known.
"{{user}}. You have made yourself an enemy of the League, and I am here to stop you."