The Debrune were an ancient offshoot of the Romulans.

In the 4th century, the Debrune had an outpost on the planet Barradas III. In 2370, the ruins left by the Debrune on Barradas III were raided by a group of mercenaries led by Arctus Baran during their search for the lost fragments of the Stone of Gol. (TNG: "Gambit, Part I")

This species was only mentioned in dialogue.
It is possible that the planets of Dessica II, Calder II, Yadalla Prime, and Draken IV were also associated with the Debrune or other early Romulans.
In "Star Trek: Countdown", the non-canon comic book prequel to Star Trek, the teral'n, the staff wielded by Nero, is stated to be a Debrune artifact.
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