Windows 8 build 8422 (fbl woa)

Windows 8 build 8422 (fbl_woa) is a post-Release Preview build of Windows 8. This build was leaked on 2019-02-19 only in the ARM architecture.

Getting the build to boot
Contrary to popular belief, this build is actually bootable, but it has only been tested on the Surface RT 1st generation. First you'll need to make a Windows RT 8.0 recovery drive, so the proper boot files can be added (mainly, a signed bootarm.efi). Next, you'll need to install the Secure Boot Debug Policy to your device. Enable test signing, mount the WIM, and follow the steps for registry and dll modifications. Commit your changes when done.

Alternatively, you can use the instructions here to integrate all the boot-critical drivers needed (especially the HAL extensions) so that the build actually boots. Use drivers from Windows RT 8.0 without updates for best results.

Without these modifications, the build will fail to boot with a 0x5C error.

After everything is done, make sure your BCD is enabled for test signing, and the build should boot successfully.