The Mindless Ones are the minions of Dormammu. As the name goes, they have no intelligence, but they do what they are told and don't speak. They live in the Dark Dimension as extensions of Dormammu himself. Mindless Ones are created from those who are willing vessels of Dormammu's power, such as Kaecilius and his Zealots.
The Mindless Ones were created by Stan Lee and Steve Ditko.