«Dast» Doctor

At

the height of the Time War, the Daleks and Time Lords spared no effort in their attempts to outflank each other. The Time Lords boasted clear supremacy in the manipulation of time and dimension, while the Daleks far outstripped them in resources, weaponry, and genetics. As the War escalated, the Time Lords seized upon a gambit: they would send the Doctor, the man most familiar with the Dalek menace, to erase their enemies from the timestream itself.

They underestimated the Daleks' counter-temporal technology. The Doctor was captured. And the Daleks seized a gambit of their own.

They rewrote the Doctor's DNA, reprogramming his genetics to make him as Dalek as he was Time Lord. They assigned him the rank of a Dalek Dast – a task force commander. And they gave him orders to destroy Gallifrey.

The result was unexpected. While unable to ignore the Dalek genetic imperative to destroy, the Doctor-Dast retained enough of himself to turn on the Daleks, ordering his task force to turn on and destroy Skaro. The Daleks under his command were unable to resist the Dalek imperative to obey, and did so.

Time Lords

To a Time Lord, the Doctor still registers as a Time Lord... mostly. There's something a bit off about him, his skin is unpleasantly hot and dry, and what minor psychic abilities he once had have been all but eliminated. He sleeps periodically – much more often than Time Lords are wont to – and, when he does so, is subject to random contractions across his entire musculature. He writhes.

His biology has been altered on a cellular level. His binary cardiovascular system has been replaced with a single, Y-shaped organ which functions much like a worm's heart. His blood is viscous, with a faintly green tinge.

Even he's not sure if he still retains the capacity to regenerate, or what the consequences might be.

Daleks

A scan will reveal the Dast to have genetic markers which identify him as a Dalek Dast. This allows him a certain influence over Dalek soldiers – most find it difficult if not impossible to disobey an order by their apparent superior.

This was somewhat counteracted by the Dalek edict after the burning of Skaro, once again naming him as the greatest enemy of the Daleks. Even so, the Dast is known to be able to command weaker-willed Daleks who cannot overcome the genetic imperatives, which makes him, as the remnant Dalek command structure knows, a dangerous quarry.