It's a alternate version of the Omnitrix.
The Carnitrix is a alternate version made by Azmuth, a evil tyrant that hated other aliens other than his.
The Carnitrix is made to destroy every single being in the universe, and you are it's user.
The Carnitrix detects anything that moves in a 20 meter distance a danger, thus transforming in it's own with a alien.
The alien is completely agressive, and you can't control it, your mind awake and the alien kills the nearest living being in instinct.