Anchoring of the thread

The anchoring of the thread was the event in which the Great Houses created history with themselves at the center 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 infinity 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 unkind to the Houses' civilization; 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 Anchoring
To permanently fix this issue, the Houses created a causal structure for the future called the Spiral Politic. The machinery needed to accomplish this task 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 center of the machine, stitching the Houses' biologies into the universe at a fundamental level. (PROSE: The Book of the War)

From then point on, 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 and drove the children of Pythia to powerlessness. However, in doing this, the Watchmakers surrendered the creative and changeable parts of their minds (PROSE: Christmas on a Rational Planet) as well as their fertility, prompting the creation of breeding engines and the Great Houses of Gallifrey to stabilize the population. (PROSE: The Book of the War, Cat's Cradle: Time's Crucible) As a result, the Homeworld began ten million years of total cultural stasis. 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) The Eternal War began. (PROSE: The Pit)