The Mandragora Helix is a living spiral of pure energy that radiates outward, and at the same time exerts a powerful force that can draw things into it. It also seems to have a powerful psychic presence and is able to infiltrate the human mind. How it functions is a complete mystery. Inside the Helix are a series of concentric crystal rings suspended in blackness. All sound echoes in this void. Dwelling within the Helix is a controlling intelligence that presumably lives on Helix energy.
A ball of Helix energy looks like an irregularly-shaped red blob that shoots out sparks. As it floats through the air, it makes a loud whining noise. Helix energy has the ability to severely burn anything that comes into contact with it through "subthermal recombination of ionized plasma" - this seems to mean that it highly ionizes whatever it touches. In the case of human skin, this results in the accumulation of some sort of blue crystalline growth on the surface.
The Mandragora Helix is worried that man, unchecked, would not be contained in his own galaxy, and might become a powerful rival. In order to prevent this, Mandragora was able to send a ball of Helix energy to Earth via the TARDIS. Besides causing fatal burns, the Helix energy allows it to dominate and control the beings it touches, and through the Helix energy, Mandragora can transfer some of its power to humans. The Helix energy completely consumes Mandragora's subjects and enables them to fire electrical-like energy bolts from their fingertips. However, a limited amount of Helix energy means a limited amount of power, and Mandragora cannot send more energy without a means of transport. Hence, its power can be exhausted.
Mandragora's plan to control humankind would result in man becoming like mindless sheep with no ambition or purpose. The alignment of constellations must be right for Mandragora to attempt a takeover, and after its attempt in the fifteenth century, the next possible time would be toward the end of the twentieth century.
