The word "Mara" describes both the species and the individual members of that species.
The Mara was a being which fed on the suffering and madness of others. It tempted individuals into letting it control them and would create havoc and chaos to pleasure itself. Something the Kinda were able to understand is that life and death are a turning wheel, and it was the Mara who turned the wheel. In this thinking, wars would help the Mara as they created death and the true way to be free of the Mara was to find peace.
The Mara was an entity created from the evil in the minds of the people of the planet Manussa in the Scrampus system. It was given independent life via the Great Crystal, which the Manussans created in a zero gravity environment. The Mara then founded the Sumaran Empire. It existed in the minds of its victims and can transfer itself in the form of a tattoo-like mark, to those who yielded to it. It was so evil that it cannot bear the sight of its own reflection.
On Manussa, the Mara was defeated and driven out by an ancestor of the future Federator and cast into the "dark places of the inside".
It was on the planet Deva Loka that the the Doctor, Tegan and Adric encountered the Mara. (Nyssa, meanwhile, rested in the TARDIS.) Tegan fell asleep listening to wind chimes and mentally entered the Dark Places of the Inside. The Mara manifested to her as the elderly game-playing couple Anatta and Anicca, who insisted she could not possibly exist, and then as Dukkha, who tempted and tormented her until she agreed to let Mara take over her body. The Mara used her body to find and possess Aris, one of the peaceful Kinda tribe, leaving Tegan. Kinda tradition did not allow men (other than "idiots") to speak. Aris called himself "Aris, He Who Has Voice" and began to rally them against Human colonists led by Hindle. The Doctor was able to prevent the humans detonating a bomb and managed to trap the Mara in a circle of mirrors and face itself, therefore, driving it back into the Dark Places of the Inside. Tegan remained very shaken by the experience, as she confided to Nyssa afterward. Unknown to all, the Mara still had influence over Tegan.
Late, the Mara guided Tegan to take the Doctor's TARDIS to Manussa, the birthplace of the Mara, where a ceremony was to be held to mark the 500th anniversary of its banishment. The Mara used Tegan, the showman Dugdale, and the son of the Federator, Lon, to obtain the Great Crystal to restore itself to physical itself. The Doctor was guided by an old mystic named Dojjen who showed him how to find the "still point". When the Mara tried to make its return at the ceremony, the Doctor concentrated his thoughts with a small replica of the Great Crystal, and by finding the still point was able to repel the Mara. Then by grabbing the Great Crystal, the Doctor broke the Mara's hold over its controlled Manussans, and destroyed its new snake body. This time, the Mara had apparently been completely destroyed for good.