This work presents a novel marriage of Swarm Robotics and Brain Computer Interface technology to produce an interface which connects a user to a swarm of robots. The proposed interface enables the user to control the swarm's size and motion employing just thoughts and eye movements. The thoughts and eye movements are recorded as electrical signals from the scalp by an off-the-shelf Electroencephalogram (EEG) headset. Signal processing techniques are used to filter out noise and decode the user's eye movements from raw signals, while a Hidden Markov Model technique is employed to decipher the user's thoughts from filtered signals. The dynamics of the robots are controlled using a swarm controller based on potential fields. The shape and motion parameters of the potential fields are modulated by the human user through the brain-swarm interface to move the robots. The method is demonstrated experimentally with a human controlling a swarm of three M3pi robots in a laboratory environment, as well as controlling a swarm of 128 robots in a computer simulation.