Using emulation software is generally legal, but downloading game ROMs or BIOS files that you do not own is a violation of copyright.
While native emulation (via a fake DXBX) is not viable, there are three legitimate ways to play Xbox games on your Android phone today. dxbx emulator android
No. To play Halo on Android, use Xbox Cloud Gaming (Game Pass) or the official Halo: Spartan Strike mobile game. You cannot emulate the original Halo: Combat Evolved natively via DXBX. Using emulation software is generally legal, but downloading