An early Earth space probe that merged with an alien probe and became a powerful machine bent on destroying any life it judged imperfect.
Nomad began as an Earth probe built by Jackson Roykirk and launched in the early 21st century to seek out new life. After it was damaged, it merged with an alien probe called Tan Ru, whose mission was to sterilize soil samples. The combined machine rebuilt itself and adopted a new purpose: to find and wipe out imperfect life.
By the time of The Changeling, Nomad had destroyed the billions of inhabitants of the Malurian system. When it met the Enterprise, it mistook Kirk for its creator Roykirk and came aboard. It erased Uhura's memory, killed Montgomery Scott and then restored him, showing its considerable abilities.
Kirk defeated it with logic, pointing out that it had erred in calling him its creator and so was imperfect itself. Unable to accept its own flaws, Nomad began to self-destruct and was beamed into space just before it exploded.