The Garvond was a monstrous psychic entity, a time-creature that was a group mind of the evil and demented thoughts of Time Lords when their minds were imprinted into the APC Net of the Matrix.
Existence
The Garvond was a creature feared to be more powerful than time itself. The Doctor, having foreseen his own part in its creation when he entered the Matrix, erased his own mind print from the Panotropic Net to avert its creation. Its rebirth was initiated by Mortimus's use of the Chronovore Artemis.
It had several thousand names, including Garivont and Garavond. All names were corruptions of the High Gallifreyan "gjara'vont" which meant "of darkest thought."
The Garvond's form resembled a skeleton cloaked in darkness, sitting upon a throne outlined in blazing fire and surrounded by dark and light tendrils.
In the deepest sense, it was highly similar to the equally malevolent Dark Matrix.
Power
It was more or less crippled at the moment of its rebirth, desperately requiring enormous amounts of time energy to replenish itself. It fed off paradoxes and could seize control of artificial devices. The Garvond was also able to inflict massive psychic damage by reaching into people's minds. It could extend itself to other items and use them to complete a circuit, as it did when it made the starship Icarus a part of it; additionally, it needed a Time Focus which connected two points in time and which it could possess.
It also made use of Time Soldiers, humanoid ghost-like creatures that had at once been humans, Gallifreyans, Tharils, and possibly other creatures who had travelled in time. They could phase out of time and appear anywhere, invulnerable to energy weapons. They were tall and had snouts that looked like gas masks; they carried wide-barreled blasters as part of their arms which fired time beams that could age/de-age their targets. Their senses were bombarded with so much information that they couldn't actually see and tracked targets by movement. Hylerium reflected their time energy and was the only effective weapon against them. The Garvond was able to speak through them as they acted as a part of its being.
History
The Worshipful and Ancient Law of Gallifrey recorded that a Cardinal wrestled with the Garvond in the Matrix and it ate away his remaining lives, killing him without hope of regeneration. The book also foretold the coming of the Garvond, after being trapped in the Matrix for millennia, imprisoned by the minds whose fear and hate fed it.
Due to Mortimus's meddling, the Garvond's existence was ensured and it remained in the Time Vortex as it gathered its Time Soldiers and grew stronger. It became allies with Epsilon Delta, a renegade Time Lord, and agreed to help his TARDIS achieve feats unimagined. Having Epsilon Delta's android Amanda assassinate a government minister during a thousand year period of "crystallised time" when he wasn't assassinated would release vast amounts of energy as the "natural" flow of time snapped back after the disruption. It wanted to feed on this and continue to ravage the fields of Time.
Surrendering his TARDIS to it, the Doctor lured the Garvond into a randomised Architectural Configuration and had it chase him into the TARDIS library. There he caught it in a book, possibly a copy of The Worshipful and Ancient Law of Gallifrey, and tossed it into a dimensional trap. It remained trapped there as long as the reconfiguration circuits were not played with. (PROSE: The Dimension Riders)