Dark Times

 Not to be confused with the The Dark Time, a period in the history of Gallifrey. 

The Dark Times was one of the earliest aeons in the history of the Universe.

Beings
The Tenth Doctor described this as a time of chaos "before our reality" when the Nestenes and Great Vampires flourished, (TV: The Infinite Quest) as well as the Carrionites. (TV: The Shakespeare Code) The Carrionites waged a war with the Hervoken. (PROSE: Forever Autumn) The Eternals were present during this time period, banishing both the Carrionites and the Hervoken to the Deep Darkness. (TV: The Shakespeare Code), (PROSE: Forever Autumn). The Racnoss were alive during this time period, with the last-named species having its defeat at the hands of the Fledgling Empires. (TV: The Runaway Bride) The Mandragora Helix also originated during this period. (PROSE: Beautiful Chaos) Krafayis evolved and began to swarm across the Universe towards the end of this period. (TV: Vincent and the Doctor) The Great Old Ones were also present in the Universe at this time. It was during this time period, too, that the legendary spaceship Infinite was created. (TV: The Infinite Quest)

War
The Time Lords fought the Great Vampires during this time and, led by Rassilon, they made Bowships to kill the beasts. They killed all but one after a war so long and bloody that the Time Lords were sickened of violence and officially began their long period of non-interference. (TV: State of Decay)

Other
Rassilon also wrote his Record of Rassilon during this period. It instructed Time Lords to kill the King Vampire if they encountered it. (TV: State of Decay)

Chronological placement
When the Dark Times actually occurred was not known. The Tenth Doctor implied that the Dark Times ended with the destruction of the Racnoss, which occurred 4.6 billion years before the 21st century. (TV: The Runaway Bride) On the other hand, the war between the Hervoken and the Carrionites was said to occur a mere million years before then. (PROSE: Forever Autumn)