The Elasi pirate frigate was an old repurposed Klingon design.
The Elasi captain Bivander Zane on the bridge of a frigate.
The Elasi frigate, also known as the Storm-class frigate, was a class of highly maneuverable combat starships used by the Elasi pirates in the 23rd century.
The class was small and armed with a variety of defensive systems. In a one-on-one engagement they were a match for older Federation light cruisers, and two working in tandem could threaten a Constitution-class starship. At least some examples possessed a cloaking device.
The model was believed by the Federation to be an altered type of early Klingon battlecruiser, although the Klingon government denied this. Between 2266 and 2269 the Elasi participated in forty-six attacks on Federation shipping, and were considered a class 1 security threat, the latest in a long line of pirates to plague those sectors that the Federation could adequately patrol given the vastness of interstellar space.
In the 2260s, the Elasi pirate clans had an alliance with the Vardaine, an isolationist human-offshoot colony that practiced illegal genetic engineering. Their frigates assisted anti-Federation isolationist Dr. Ies Bredell in his plans to covertly construct a fleet of Constitution-class vessels for the Vardaine, beaming him to safety when his first production was defeated by the USS Enterprise. Later, they were present when Bredell tried to use a device known as the Big Bang Weapon to destroy the Federation in revenge. (TOS video games: 25th Anniversary, Judgment Rites)