The Empty Ones were a small and short-lived cult based in Nevarra and known for worshipping the blight and, by extension, the darkspawn. Some confuse the Empty Ones with followers of Tevinter's Old Gods—a reasonable mistake since Archdemons are said to be tainted Old Gods. However, it is clear from the histories that the Empty Ones did not worship Dumat and his ilk, but the blight itself.
Following Andraste's death, many of her followers fell into a deep despair. They believed that the Prophet's betrayal and execution marked the beginning of the end of the world and that the Maker's wrath would soon come upon them. The most fatalistic of them all gathered together to prepare for their doom. They called themselves the Empty Ones, for they saw themselves as worthless husks, ready to be swept away by the Maker's hand.
It is unknown what passed then, but over time, the Empty Ones grew to believe that the blight was to be the tool by with the Maker would end all of creation. They preached that it came from the Void, a place of nothing, and that returning to the Void was something to be celebrated because it meant an end to all pain and all suffering.
Some mistakenly take this to mean that the Empty Ones worshipped evil, but that is an oversimplification. The Empty Ones believed the world to be beyond redemption, and that it was the Maker's will that it be destroyed completely. There are tales of Empty Ones scouring the Deep Roads, searching for darkspawn, whom they saw as the blight's prophets in order to assist them in bringing about the next Blight.
Predictably, the beginning of the Second Blight saw the end of the Empty Ones. The entire cult made its way to the Anderfels, where they stood in the path of the encroaching darkspawn and, singing in praise of the oblivion that was to overtake them, were consumed.