How can I create this shape in Flutter?

You can use the Stack widget and it's child widgets, it has a container with a circle, after which you can push the circle off the screen so that only part of it is rendered

class _MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: MyApp._title,
      home: Scaffold(
          body: Column(
        children: [
          CircleCorner(),
        ],
      )),
    );
  }
}

class CircleCorner extends StatelessWidget {
  const CircleCorner({
    Key? key,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    Size size = MediaQuery.of(context).size;
    return SizedBox(
      width: size.width,
      height: 300,
      child: Stack(
        children: [
          Positioned(
            top: -460,
            right: -380,
            child: Container(
               width: 600.0,
               height: 600.0,
              decoration: new BoxDecoration(
                color: Colors.black87,
                shape: BoxShape.circle,
              ),
            ),
          ),
        ],
      ),
    );
  }
}