The Graak

The Graak was an amorphous being created by the Fourth Doctor and K9 to rescue his first seven selves from.

Creation
The Graak was created by the Fourth Doctor and K9 Mark II in the emergency of combating any sort of psychic attack against the TARDIS or the Doctor by either the Master or the Black Guardian. The Doctor gave the Graak full movement capabilities and the ability to keep in touch with all of the Doctors via a telepathic link. (GAME: Destiny of the Doctors)

Freeing the Doctor
The Graak went on his mission to free the Doctor almost as soon as he was created. Starting in the Fourth Doctor's TARDIS, he set about retrieving various objects to get across the Great Divide, after which he would free the Doctor who was in the "time zone" he currently occupied.

Throughout his quest, the Graak was mocked endlessly by the Master, who tried various means to stop him from accomplishing his tasks. The Master even attempted to hypnotise him at least twice.

After completing several of the Master's quests and mental challenges, the Graak finally encountered the seven Doctors trapped at the Heart of the TARDIS.

The Master had sent the Doctors into the time vortex, and they had ended up in the Heart itself. The Graak's energy levels were waning, and the TARDIS was on the brink of destruction.

Death
Making his way through water-filled caves and into the Heart, the Graak used up his remaining life force to materialise the Doctors into the TARDIS, saving the Doctors and the TARDIS at the cost of his own life. (GAME: Destiny of the Doctors)

Personality and abilities
The Graak displayed no clear personality or emotions of his own, though he followed his purpose according to the Doctors' instructions. During the final battle against the Master he willingly sacrificed his remaining life energy to save the Doctor, showing a capacity for heroism.

Although he floated just above the floor without any visible legs, the Graak was capable of basic movement such as running, jumping and crouching. He was also able to swim, though he did so with a slight degree of clumsiness.

The Graak could pick up and use various items. He could operate the TARDIS console, and access the TARDIS logs, as well as fly the TARDIS between time zones.

The Graak could transfer his consciousness to security cameras placed throughout the TARDIS to get a different view of his surroundings.

He was able to channel massive amounts of energy on a single spot, but this rapidly drained his life force. (GAME: Destiny of the Doctors)

Appearance
The Graak was a translucent, amorphous being with a blob-like body in which glowing orbs floated, two pseudopod-like arms made up of strands of the orbs, and one large orb serving as what the Master called a "potato-like head." Though technically a genderless being, the Graak was generally referred to in the masculine by the Doctor, the Master, and the Brigadier.

Behind the scenes

 * Destiny of the Doctors is primarily played from the Graak's first-person point of view. The Graak himself is only able to be seen using the security cameras in certain parts of the Doctor's TARDIS.
 * Though it's not specified which version of K9 helped to create the Graak, it's assumed to be Mark II due to the mention of combating against the Black Guardian.