The Sentinels of the New Dawn were a secret society active on Earth at least from the 20th century to the 22nd century. They were compared by the Doctor to the Illuminati. They were powerful people who conspired to become even more powerful. (AUDIO: The Sentinels of the New Dawn, Leviathan)
In the 21st century, they were encountered by the Third Doctor and Liz Shaw (who'd been hurled through time). The Sentinels were using a time window to make themselves wealthy, peering at the stock markets of the near future, and intended to cause an ebola epidemic to take power. Liz Shaw ensured they wouldn't have access to time travel technology anymore. (AUDIO: The Sentinels of the New Dawn)
By the 22nd century, they'd become increasingly dangerous and were responsible for creating a biomechanoid enforcer, modelled on Herne the Hunter, to murder their enemies. The Earth governments united against them and forced the Sentinels to flee from Earth. They stole a prototype Leviathan which they populated with clones and android overseers — with a Zeron AI running it — and set on a course towards the planet Phlegethon. The Sentinels remained in cryogenic sleep; the clones lived in a fake medieval world, being "recycled" before they grew too old to question authority and their nutrients were used to feed the Sentinels.
The Leviathan was hit by a meteor storm, damaged, and floated off course for centuries. By the time the Sixth Doctor and Peri Brown came across it, the Sentinels had all died as their stasis pods failed. (AUDIO: Leviathan)