D'era, also known as the Way of D'era, was the guiding belief of the Romulan people that was developed when they settled on Romulus. It was created by Tellus who helped shape the Romulan Star Empire.
The teachings of D'era came as a response to the logic preached by Surak on the Vulcan homeworld during the Time of Awakening. D'era itself translates as "Endless Sky" with the ancient religions of early Vulcan resulting in a new set of beliefs for the developing race. D'era itself teaches the Romulan people that they and the Vulcan people were the children of the Vhorani people who journeyed from distant Vorta Vor to the Vulcan homeworld to temper the race. The philosophy stated that that the reason for this was because the harsh environment would temper the race to make them strong and worthy inheritors of the Vhorani. This essentially gave the Romulan people a racial destiny as the rulers of the universe. It became popular among those Vulcans who refused the pacifistic ways preached by Surak but did not become a dominant belief which forced its followers into exile from the homeworld. These exiled people believed their kin had betrayed their people's manifest destiny as caretakers of the universe.
As such, it became a series of guiding principles for the Romulan way of life and determined how the race were to react with one another and view other races. This developed a psychological urge to complete this destiny which developed both their military as well as government. It also became the basis of their religion which justified their peoples violent war-like ways. It also became the source of their arrogant superior attitude towards other races.
Though it preaches of a higher destiny for their people, D'era teaches more then this as it tells its followers teachings of obedience, discipline and other similar traits that keep the Romulan people, and their Empire, a unified force. Without which, the race would consume themselves in the fires of internal conflict and destroy themselves. As such, it has an important driving force within Romulan culture which is taught when every Romulan is born.
Aspects of D'era are covered by the Virtues of the Way. (Last Unicorn RPG module: The Way of D'era: The Romulan Star Empire)