- You may be looking for the Koturians.
The Kotturuh were a species from the Dark Times who could apply a limit to the lifespan of other species, or destroy them outright if they wished, as part of their "Great Task" in accordance with the "Great Song" in the "Design". They were ultimately rendered extinct by the actions of the Tenth Doctor during what would be remembered as the Kotturuh crisis.
The Kotturuh were immortal, with the ability to grant and 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, The Dawn of the Kotturuh)
The Tenth Doctor once saw beneath a Kotturuh's veil. Their face was covered in deeply-etched symbols which were bleeding and their eyes oozed rot. The sight filled him with "a primal horror". (PROSE: The Knight, The Fool and The Dead)
The Kotturuh saw death as both their religion and their gift to other species. (PROSE: The Guide to the Dark Times) They referred to this as their "Great Task". (PROSE: The Dawn of the Kotturuh) They assigned lifespans according to how species fit in their "Design", (PROSE: The Knight, The Fool and The Dead) which they also referred to as the "Great Song". (PROSE: The Dawn of the Kotturuh)
The Kotturah often acted polite when interacting with those they judged, but would still saw them as beneath them, speaking over them once they had decided to communicate and giving off information without consideration to how they would take it. (PROSE: The Dawn of the Kotturuh) If they felt they had been welcomed accordingly, the Kotturuh would sometimes give those they judged the option not to be given death. (AUDIO: Lesser Evils)
When the Unbound Doctor was planning on blowing up the Ark of Whispers to prevent it from crashing into a planet, Bernice Summerfield mentioned the rites of the Kotturuh as a cultural artefact that might be preserved 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 hailed from a realm which could be accessed via a gateway on the planet Mordeela. (PROSE: The Knight, The Fool and The Dead) They were among the first species in the universe. (PROSE: The Guide to the Dark Times)
They believed that they served the "Ruler of the Universes" (PROSE: All Flesh is Grass) and that they had been spreading the "gift" of death in the universes before N-Space, and would continue to do so in the subsequent universes. (PROSE: The Dawn of the Kotturuh)
However, according to Inyit, the Kotturuh's beliefs were wrong. They had actually just become imbued with the death energies entering the universe at Mordeela, but those energies were entirely natural and would have spread into the universe anyway without the Kotturuh. (PROSE: All Flesh is Grass)
After "a millennium or two", the Kotturuh set out to distribute death among the denizens of N-Space, (PROSE: The Guide to the Dark Times) visiting worlds peacefully before allocating lifespans to the natives of that world, or simply destroying life on a planet without so much as a word. (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 assigned them a lifespan of three months. (PROSE: The Dawn of the Kotturuh) One Kotturuh, Inyit, turned against their crusade on Birinji, sparking a conflict within the Kotturuh that left Birinji a dead world. Inyit was cast out as a result. (PROSE: All Flesh is Grass)
The Kotturuh kidnapped individuals from across the universe to turn them into revenants, undead forced to recite the date in the Design when their people should be judged. Numerous delegates who travelled to Mordeela to try to negotiate with the Kotturuh were also used for this purpose. (PROSE: The Knight, The Fool and The Dead) 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, while the Great Vampires managed to keep their immortality on the one condition that they must feed on others to sustain their existence. The Dæmons tried to fight back, but the Kotturuh were too powerful, leaving only Azal, who fled to Earth to live in suspended animation. (PROSE: The Guide to the Dark Times)
The Kotturuh made the natives of Eloii sterile and short-lived so that one of their "preferred races" could one day colonise the planet. Hana Fallomax was the only survivor, having been wearing armour created by her father using crystals from a meteor he believed came from Mordeela. When charged the crystals could negate the Kotturuh's energy wave, but frequently needed replacing. As a Professor, Fallomax dedicated herself to researching a way to make a fully working lifeshroud.
The Kotturuh visited the planet Destran, consigning it to death, save for Estinee, who possessed an immunity to their touch. They kidnapped Estinee and brought her to Mordeela, but she was rescued by Fallomax, who hoped to use her immunity to help develop lifeshrouds. Their work became little more than a scam as Fallomax had Estinee demonstrate non-functional lifeshrouds to sell them on worlds about to be judged by the Kotturuh, exploiting Estinee's immunity to death to make them appear to work. Fallomax was unable to make the lifeshrouds last more than a few hours as they relied on the Mordeela crystals which burnt out after a few hours. At one point, the Kotturuh recaptured Estinee, but Fallomax rescued her a second time.
In the original version of history, the Kotturuh continued to exist for billions of years (PROSE: All Flesh is Grass) into the Doctor's era. (PROSE: The Knight, The Fool and The Dead; AUDIO: The Enemy of My Enemy) When the Master found himself exiled on the planet Alexis, he offered his services to a Kotturuh woman as an advisor on dispensing death, but was rejected. (AUDIO: Lesser Evils) The Tenth Doctor was aware they still existed, though he had never encountered them. (PROSE: The Knight, The Fool and The Dead)
When the TARDIS, which knew the Doctor had to make a major mistake and learn from it, brought the Tenth Doctor into the Dark Times via a Time Fracture opened up by the brief survival of Adelaide Brooke, (PROSE: What the TARDIS thought of the "Time Lord Victorious") the Kotturuh came to the planet Andalia and gave its citizens a lifespan of a single lunar orbit as the Doctor was visiting, whom they quickly made an enemy out of. The Kotturuh also recaptured Estinee, who had been demonstrating Fallomax's lifeshrouds, prompting the Doctor and Brian the Ood to follow them back to Mordeela to rescue her. Estinee was rescued again by Fallomax and the Kotturuh confronted the Doctor in his TARDIS, marking him to die on Andalia in a few days, which only further inspired the Doctor to stand against them. Though he discovered Fallomax's Lifeshrouds were a fraud and couldn't be made to work longer than a couple of hours, he was able to use genetic material from the Kotturuh physically attacking him to reverse-engineer their power as necrotic virus.
Returning to Andalia to analyse energies left by their judgement, he devised two solutions; one to give them a lifespan, and another to wipe them out by only allowing them to live for fifteen minutes. The Kotturuh used their influence over Estinee to have her move the TARDIS to the Doctor's allotted date of death and also had her kill Fallomax. They confronted the Doctor and attempted to make Estinee shoot him, but she refused and used the gun to detonate her lifeshroud instead. The explosion caused the Doctor to glimpse beneath a Kotturuh's veil and broke the vial holding his lesser solution. Infuriated at their actions, he unleashed his second solution, giving them a lifespan of fifteen minutes. The Kotturuh on Andalia then crumbled to dust before his eyes.
The Doctor's virus spread through the Kotturuh, resulting in numerous ships fleeing to Mordeela in the hopes of its barrier delaying their fate, as they broadcasted to the mercenary fleet as it approached Mordeela, begging the Doctor for mercy. He refused and ordered his fleet to attack Mordeela to seal their realm for good. The Eighth and Ninth Doctors arrived above Mordeela allied with a Dalek Time Squad and a coffin ship of the Free Undead to plead with the Tenth Doctor from going through with the final attack against them. (PROSE: The Knight, The Fool and The Dead) The Tenth Doctor ignored their pleas and his fleet fired on Mordeela, destroying the planet and instigating the Battle of Mordeela. Despite the loss of all his forces except one ship, the Tenth Doctor was able to carry out his plan of using Mordeela's ruins to plug the gateway before fleeing. (PROSE: All Flesh is Grass)
One ship of Kotturuh led by Avkhan returned to Magnox, demanding the Minds of Magnox tell them how to survive their death and defeat the Doctor. They offered to end their judgement on Magnox in return and threatened to wipe the Magnoxians out if they refused. The Minds asked Avkhan to ask them in person, but, when he arrived in the Chronicon, Avkhan found the Minds had been killed by Brian and was confronted by the Doctor. The Doctor used a data field to incapacitate him and then used the Minds' technology to temporarily halt the Kotturuh ship so he could order an evacuation of the planet before they wiped the people out. (AUDIO: The Minds of Magnox)
The few surviving Kotturuh rededicated themselves to carrying out judgements before they succumbed to the Doctor's virus. They were hunted down by the Dalek Time Squad, who feared their judgement on the Daleks. Eventually, the Kotturuh were all dead except for Inyit, who returned to Birinji to await her death where she was visited by the Ninth Doctor and Ikalla. The Daleks sent a scout ship to Birinji, but it was destroyed by the HMS Donna commanded by the Tenth and Eighth Doctors. Meeting Inyit forced the Tenth Doctor to reconsider his views on the Kotturuh. Whilst the Doctors went to defend Gallifrey from the Time Squad, the original Dalek Symbiont was sent to kill Inyit. However, she used the remaining energies to issue a final judgement, wiping out the Symbiont and all the Dalek Drones that had been given its DNA. In doing so, she forced the Daleks to stop their attack on Gallifrey and flee from her judgment. Having used the last of her energies, Inyit died, rendering the Kotturuh extinct, though passed on some of her energies to Ikalla. After witnessing her death, the Tenth Doctor admitted he had made a mistake.
The Wraxians discovered technology left behind by the Kotturuh and used it to make the Devolver. Similar to the Kotturuh's gift of death, the Devolver was able to alter the lifelines of entire species at once by influencing biodata. (AUDIO: The Enemy of My Enemy) The Master later discovered the Devolver and created a handheld personal weapon based on it. (AUDIO: Master Thief)