Alkhema is a creation of Ultron, as well as his second bride. While Natasha Romanov was still his prisoner, while she was unconsciousness he used her as a test subject for his plan to create a companion. He then downloaded Natasha's mind and put in in a secure file while he planned to wipe out the human race in Sokovia.
Years later, after his first attempt to create a companion failed with Jocasta joining the Avengers. Ultron chose to use the Black Widow's brain patterns in create his second bride, and designated her Alkhema. After her AI and robotic body were complete Alkhema was programmed with additional fail safes to prevent the same problems that happened with Jocasta. After which it seemed as if Alkhema had Natasha's heartless personality while she was still with the KGB. Alkhema then became the true bride of Ultron and she would work with him to make the world "clean for the new man to be born".
Alkhema is a superhumanly strong and phenomenally durable robot composed of Vibranium and outfitted with a variety of offensive weapons.