The Infinite Realm

"The Infinite Realm" is the twelfth and final episode in the eighth season of Doctor Who, which sees the culmination of the events in Into the Void, and indeed majority of the series. It is the last episode to feature Isabella Cartwright, who is killed by Omega, the first companion death in years.

Synopsis
"The words have followed you Doctor, one will die! One will die! And I will be the one to destroy you!" - Omega

The Doctor and his companions are powerless to stop the mysterious planet from hurtling into an anti-matter dimension, the realm of the Time Lord Omega, seeking revenge on the Time Lords and the Doctor, and also to re-assert his god-like power. As the planet transports into the anti-matter realm the Doctor struggles to stop Omega, but is helpless as an anti-matter horde begins to pour onto the planet, to rejoin with their master Omega. And as the Doctor was told by the White Guardian, one will die.