Anchoring of the thread

The anchoring of the thread was the event in which the Great Houses created history with themselves at the centre and accidentally unleashed the Yssgaroth. (PROSE: The Book of the War) The Seventh Doctor described this as Rassilon's decision to make the universe rational. (PROSE: So Vile a Sin)

Motivation
Before the anchoring of the thread, the universe was unstructured and chaotic. (PROSE: The Book of the War) There were no laws of physics, only infinite possibility. (PROSE: Christmas on a Rational Planet) However, the Great Houses predicted that, as new cultures emerged and began to impose their own understandings and versions of meaning onto the continuum, a definite framework would emerge. Since these other species could be completely different to the Houses, their assembled history might be inhospitable to the Houses' civilisation; indeed, some members of the Houses had already glimpsed unclassifiable events in the formative future. The earliest time technologies were developed and used to avert such events. (PROSE: The Book of the War)

The time before the laying of the thread was the Dvapara Yuga. During this time, humanity had divine powers: for instance, Arjuna had one thousand arms and was ardently skilled at compressing time, while Nakula and Sahadeva led armies of imaginary hybrid animals made of broken time. (PROSE: Dharmayuddha)

The Anchoring
To permanently fix this issue, Rassilon decided to make the universe rational. (PROSE: So Vile a Sin) To achieve this, the Houses created a causal structure for the future, called the Spiral Politic. The machinery needed to build it was bigger than anything else ever built on the Homeworld. Exploratory proto-timeships attached themselves to strategic points in the future to anchor in place the structure of history being imposed from the Homeworld. At the ceremonial locking-in of the mechanisms, elite representatives of the six ruling Houses bonded their Houses at the centre of the machine, stitching the Houses' biologies into the universe at a fundamental level. (PROSE: The Book of the War) The Eye of Harmony played an important role as the "hitching post of chronology". (AUDIO: Neverland)

According to some obscure legends, the Great Houses preserved a small sliver of irrational reality, the 2nd Second, so that they could reverse the anchoring if it did not go as planned. (PROSE: Weapons Grade Snake Oil)

The gods' plan for ending the Dvapara Yuga and beginning the Kaliyuga involved a war between the Pandavas and the Kauravas; specifically, it required Krishna to be killed by an archer. King Jarasandha foresaw this and tried to ensure Krishna would die of old age, but the gods intervened and history was successfully rewritten. (PROSE: Dharmayuddha)

After the anchoring, the Homeworld was set apart from the rest of the universe as an observer and definer of the Spiral Politic. The Houses' protocols, such as linearity, became laws of physics itself. (PROSE: The Book of the War) The universe's infinite possibility was limited, the Watchmakers instead defining what was rational. This caused whole civilisations to disappear, (PROSE: Christmas on a Rational Planet) as Rassilon now had the power to choose what species developed; he exercised this in his imprisonment of the Divergence, who he saw as a possible rival to the Time Lords. (AUDIO: Zagreus) The imposition of the Web of Time also drove the children of Pythia to powerlessness. (PROSE: Cat's Cradle: Time's Crucible, The Pit, Christmas on a Rational Planet)

However, in doing this, the Watchmakers surrendered the creative and changeable parts of their souls, which collected into a cloud in Gallifrey's atmosphere before being banished into the Time Vortex by the first King of the Majestic Clockwork. (PROSE: Christmas on a Rational Planet)

At the same time, (PROSE: The Book of the War) the Time Lords lost their fertility, prompting the creation of the Looms and the Great Houses of Gallifrey to stabilise the population. (PROSE: Cat's Cradle: Time's Crucible) As a result, the Homeworld began ten million years of total cultural stasis. (PROSE: Unnatural History, The Book of the War) In exchange, they were indestructible, with their meta-structure of history preventing the creation of rival biological forms. (PROSE: The Book of the War)

Immediate effects
In the wake of the anchoring, multiple parties fled the Homeworld in the First Diaspora. These included the Eremites (PROSE: The Book of the War) and the children of Pythia. (PROSE: Christmas on a Rational Planet) Many notable Great Houses from before the anchoring, like Catherion and Ixion, went into a slow decay that would last millions of years.

On the day of the anchoring, the Yssgaroth escaped into the universe, destroying the site of the machinery and creating the caldera. (PROSE: The Book of the War) This began the Eternal War. (PROSE: The Pit)

Behind the scenes

 * The Eremites called the Great Houses' founder "Urizen", a reference to the in . In Blake's famous design , Urizen is portrayed as a bearded old man simultaneously creating and constraining the universe with a compass. Urizen's enemy is, the embodiment of imagination.