The Kotturuh were immortal, with the ability to grant (or enforce) a defined lifespan upon an entire species - from thousands of years to instant destruction. They typically wore robes which concealed their tentacled lower halves. (PROSE: The Guide to the Dark Times, et al.)
When the Doctor was planning on blowing up the Ark of Whispers to prevent it from crashing into a planet, Benny mentioned the Rites of the Kotturuh as a piece that may be aboard the mythical ship. (AUDIO: Have I Told You Lately?)
Kotturuh individuals wore robes adorned with symbols each taken from a different civilisation they had judged. (AUDIO: Lesser Evils) The Tenth Doctor noted these symbols seemed to shift as he tried to take them in. (PROSE: The Knight, The Fool and The Dead)
The Kotturuh believed that they had been spreading the "gift" of death in the universes before N-Space and that they would continue to do so in the subsequent universes. (PROSE: The Dawn of the Kotturuh) They hailed from a realm which could be accessed via a gateway on the planet Mordeela. (PROSE: The Knight, The Fool and The Dead)
The Kotturuh were among the first species in the universe. After a while, they set out to distribute death among the denizens of N-Space. (PROSE: The Guide to the Dark Times) This began on the planet Birinji, where a triumvirate of Kotturuh assessed the native population as contributing little to the Great Song and accordingly assigned them a lifespan of three months. (PROSE: The Dawn of the Kotturuh)
They were known to either visit worlds peacefully, before allocating lifespans to the natives of that world, or else they would simply destroy life on a planet without so much as a word.
Many species attempted to flee from the Kotturuh, such as the Eternals, who sacrificed their ability to lead their own lives in the process, hence their reliance on Ephemerals. The Racnoss, meanwhile, went into hibernation to avoid their destruction.
Other species attempted to negotiate - the Osiran Sutekh bargained with the Kotturuh for the gift of death; the Great Vampires managed to keep their immortality on the one condition that they must feed on others to sustain their existence.
When the Master found himself on the planet Alexis in the Dark Times, he offered his services to a Kotturuh woman as an advisor on dispensing death, highlighting the complex intelligence of the natives despite their simple appearance, but was rejected. (AUDIO: Lesser Evils)
The Kotturuh eventually visited the planet Destran, consigning it to death, save for Estinee who possessed an immunity to death. They also visited Gallifrey and, aware of the eventual rise of the Time Lords, deemed the Gallifreyans as having much to offer.
The Kotturuh came to the planet Andalia and gave its citizens a lifespan of a single lunar orbit as the Tenth Doctor was visiting, whom they quickly made an enemy out of. When the Kotturuh had taken Estinee, the Doctor followed them back to Mordeela to rescue her. Physical contact with one of the Kotturuh had given him enough genetic material to reverse-engineer the Kotturuh's necrotic virus and have it target them, giving them a lifespan of fifteen minutes. As they died and begged the Doctor for mercy, the Eighth and Ninth Doctors arrived to bar their future self from going through with a final attack against them. (PROSE: The Knight, The Fool and The Dead)