Narendra Agreement

The Narendra Agreement was a diplomatic treaty signed by the Romulan Star Empire and the Klingon Empire which resulted in the formation of the Klingon-Romulan alliance. The treaty binded both powers politically as well as technologically allowing the Romulans access to Klingon starships as well as warp drive technology while the Klingons recieved cloaking devices from the treaty.

The treaty was eventually abandoned when the Klingons and Romulans became enemies.

(Last Unicorn RPG: The Way of D'Era: The Romulan Star Empire)