The Omnitrix is a device that allows the user to transform into different alien beings.
The Omnitrix is an alien device that allows it's owner to transform into various aliens by fusing the DNA of an alien with the DNA of it's owner. While the device was created by Azmuth to be a guide to the universe's various alien beings, some see it's power as a weapon that could be used to wreak havoc on the universe. Ben Tennyson was the owner of the Omnitrix until Vilgax took it from him. Ben then had it self destruct so Vilgax couldn't use it to conquer the Earth.