The Etra Prime Incident was the Daleks' first attempted invasion of Gallifrey. (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)
The incident began when the Daleks removed the planet Etra Prime from space-time, kidnapping a Monan and Time Lord delegation including Lady President Romana. The Daleks removed Etra Prime so that they could harvest the Apocalypse Element from Etra Prime itself.
Twenty years later 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 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. The squad of Daleks who first followed the Time Lords landed on Gallifrey with a larger force set to follow later. The Sixth Doctor halted their advance through the Panopticon by locking all of Gallifrey's security systems to 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 create a new powerbase. 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)