Etra Prime incident

The Etra Prime Incident, (PROSE: Meet the Doctor) also known as simply the Dalek invasion to the Time Lords themselves, (PROSE: The Ancestor Cell) was the Daleks' first attempted invasion of Gallifrey, masterminded by the Dalek Emperor and carried out under the leadership of a Black Dalek. (AUDIO: The Apocalypse Element)

It indicated that the time had come for the Time Lords to abandon their non-interference policy, foreshadowing the War in Heaven (PROSE: The Ancestor Cell) and beginning the escalation of hostilities between the Daleks and the Time Lords. (PROSE: The Stranger, A Brief History of Time Lords)

According to one account, "some said" that the Etra Prime incident was what put an end to previous attempts at pacifying negotiations between the two species, such as the earlier Act of Master Restitution. (PROSE: Meet the Doctor) Post-Time War historians believed that the Etra Prime Incident was orchestrated by a new Emperor succeeding Davros, who had lost control of his newly formed Empire after he abandoned his doomed mothership during the Shoreditch Incident. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)

First hostilities
The incident began in Rassilon Era date 2776.2 (AUDIO: Neverland) when the Daleks removed the planet Etra Prime from space-time, kidnapping 300 delegates from numerous temporal powers, including the Monan Host and Time Lords including Lady President Romana, who were investigating unusual elements discovered on the planetoid. The Daleks removed Etra Prime so that they could harvest the Apocalypse Element from Etra Prime itself, using the delegates as slaves. A caretaker President took over in Romana‘s absence. (AUDIO: The Apocalypse Element)

Involvement of the Doctor
Twenty years later, in 2796.8, (AUDIO: Neverland) the Daleks set Etra Prime on a collision course with the planet Archetryx, which was hosting a temporal treaty attended by twenty member races of the Temporal Powers. The Sixth Doctor arrived at the conference, due to his TARDIS being caught in Etra Prime’s wake. The Daleks attacked the conference to gain access to the Monan Host vessel to finish their synthesis of the Apocalypse Element. Before Etra Prime collided with Archetryx, President Romana escaped from Etra Prime and warned the Time Lords of what the Daleks were doing, allowing them to retreat to Gallifrey.

The Daleks followed the Time Lords back to Gallifrey posing as a Monan Host fleet. The Time Lords lowered the transduction barriers, allowing the Daleks to invade, as Coordinator Vansell wanted to exploit the crisis to study Monan timeships. The squad of Daleks who first followed the Time Lords landed on Gallifrey with a larger force set to follow later. The Doctor halted their advance through the Panopticon by locking all of Gallifrey's security systems to his human companion Evelyn Smythe's retina pattern. The Daleks ignited the Apocalypse Element in the Seriphia Galaxy but the explosion moved faster than expected. The Doctor and Romana tried to use Eye of Harmony to control the progress of the explosion before it engulfed the whole universe, but they couldn't.

It was discovered that the Daleks wanted to seize the Eye of Harmony for the same reasons and tricked the interim Lord President to do this. The Daleks used the crystals they had installed on their casings to transfer there life essence into the Eye to give it the power boost to contain the explosion. Before all of the Daleks died, they set the time fields inside the containment field to accelerate in order to recreate Seriphia. Thus, the galaxy had been reborn, but only to serve as a powerbase for further Dalek conquest. The Time Lords restored the transduction barriers before any more Daleks arrived. Romana told the Doctor that the Time Lords would not tolerate a Dalek empire on their doorstep and wouldn't let them invade Seriphia. (AUDIO: The Apocalypse Element)

Aftermath and legacy
The Daleks seized control of reborn Seriphia Galaxy and used it as a powerbase to launch the Second Great Dalek Occupation. (AUDIO: Invasion of the Daleks) After the Great Catastrophe that resulted from that war, it was in Seriphia that the Daleks rebuilt. (AUDIO: Dalek War: Chapter Four, The Demons) The Time Lords later launched an assault on the galaxy during the Time War. (AUDIO: Hostiles)

The Etra Prime Incident was remembered as one of the events that escalated hostilities between the Daleks and the Time Lords, (PROSE: A Brief History of Time Lords) as Rojan learned during the Last Great Time War. (PROSE: The Stranger) The Daleks would subsequently invade Gallifrey again in the wake of the Gallifreyan Civil War and the subsequent Dogma Virus pandemic they'd engineered, only to be foiled by Romana again. (AUDIO: Ascension) Eventually their escalating attacks on the Temporal Powers forced the Time Lords to declare the Time War against them. (AUDIO: Celestial Intervention)

The Daleks would continue to refer to Romana by the prisoner designation they'd given her during the incident. (AUDIO: Extermination, Desperate Measures, Homecoming) She in turn retained a grudge against them, with the Eighth Doctor noting she'd likely let the Emperor's fleet that he'd trapped in a time loop stew for a while, before inevitably releasing them due to their vital role in the Web of Time. (AUDIO: Neverland) Admiral Valerian later held Romana's imprisonment against her in a presidential debate by arguing they needed a leader who fought rather than a prisoner, to her fury. (AUDIO: Desperate Measures)