Alsions had the capacity to use adaptive metamorphosis to try and change their shape, as Mordrega did when she tried, unsuccessfully, to change into human form. They could fly, and were psychically dependent and connected to the Coronoa of Alsion, which granted them powers. Their actual appearance was cone-like and pointed. They were described as like "two angels in battle" when Mordrega and Palito fought. (AUDIO: The Ghosts of Gralstead)
The Alsions considered the Corona of Alsion to be a holy relic and a sacred object, wrought from the Iqour, called the lifeforce of the Alsion's god according to Mordrega. (AUDIO: The Ghosts of Gralstead)
The Corona of Alsion was itself alive, and had for centuries been playing the Alsions, itself actually being a god, testing the armies of darkness and light in challenges.
In 1850 Palito the Noble fell to earth through a portal above Graltstead House, containing with it the Corona of Alsion. He was badly wounded, and was placed in a coffin to regenrate in suspended hibernation, along with the Corona. However the coffin was disturbed, and Palito fell into dust, his bones becoming hollow and rotten.
Mordrega rose up against the "revered leader" of Alsion, described by Mordrega in terms regarding it as a war in heaven. She was defeated, smote by his sword, and fell to Earth wounded. She tried to reorganise her form into a human, and with help from Edward Scrimescer, managed to feed on enough brains to keep on living.
Eventually Palito was revived by a link of the Corona, and did battle with Mordrega over Graltstead House, where he was defeated. But Mordrega was dragged back through the portal to Alsion along with the Corona, where the Fourth Doctor had loosened enough links to ensure its usefulness was limited, to ensure her reign of terror would not happen. (AUDIO: The Ghosts of Gralstead)