The Malus had the appearance of a large stone face with emerald eyes and a sharp, tooth-filled mouth that belched smoke. In a psychic-projection of itself, the Malus had more to its head and a reptillian body.
Because of the type of psychic energy the Malus used, it required a focus point, a medium that would generate the hate and rage. The Malus could create psychic projections with substance, effectively projecting matter from the past, which may have enabled a connection between two time zones that would allow an actual person to travel through time. The enormous amounts of energy this required were collected from energy gathering points around the area. This energy could be cut off by technological means, i.e. a TARDIS, or the focus point could be destroyed, preventing the Malus from using any more energy. If there are no more resorts, the Malus could self-destruct so as to destroy as much as it could.
The Malus appeared in a great wind (psychic disturbance) before Cavaliers and Roundheads at Little Hodcombe during the English Civil War. Will Chandler considered Malus the God of War. It was associated with apparitions, likely due to the projections, and its image was carved into the church as a representation of the Devil.
The Malus was sent to Earth in a computer-controlled reconaissance vehicle, made partly from tinclavic, and laid dormant until the 17th century. It awoke in 1643 when the English Civil War came to Little Hodcombe and began using the psychic energy generated by the negative emotions and bloodshed. The Malus became dormant again once the violence subsided.
In 1984, Andrew Verney was studying the Malus and discovered it, but Sir George Hutchinson's deranged mind caused it to reawaken. The creature took control of Hutchinson for use as a focus point. Hutchinson carried out its orders in the belief he could control the Malus and would be granted great power. He used the re-enactments of the war games to generate psychic energy and sought to recreate the bloodshed of the original battle.
The Fifth Doctor used his TARDIS to cut of the Malus' energy and made Hutchinson doubt the Malus by making him realise what he was doing to his friends. Frightened, Will pushed Hutchinson into the creature, killing him. In a final effort to destroy as much as it could, the Malus destroyed itself and the church it was in, but the Doctor, Tegan, Turlough, and their associates escaped in the TARDIS. (TV: The Awakening)