Warning: The following text contains spoilers relating to the plot of Prophecies.
The Unseen Ones are the race of powerful creatures that rule over the White Mantle and are worshipped as their gods. They set out the laws that the Mantle must follow and only show themselves to those ranked highest in the order. They are called Unseen Ones by the Mantle, but their name is revealed to be the Mursaat, by Glint and the Seer.
The Unseen Ones stepped in to save Kryta from the Charr invasion when Saul D'Alessio stumbled upon their homeland while lost in a forest during his exile. In exchange, Saul founded the White Mantle and instituted the worship of the Unseen Ones as the religion of the ruling regime and began to call on people to abandon worshipping the Old Gods.
It is revealed that the Unseen Ones had their own nefarious purposes for interfering when they did. Read the details in the article Mursaat.