Talarian warships armed with X-ray lasers
X-ray lasers were a type of ship-mounted weapon utilizing X-rays in the form of a laser emission. X-ray lasers were used by the Talarians in the 24th century.
In 2367, the captain of the Talarian warship Q'Maire, Endar, powered up the weapons of his ship, neutral particle weapons, X-ray lasers and merculite rockets in order to "convince" the crew of the USS Enterprise-D to return his adopted Human son, Jono. The weapons were however not a match for the defenses of Federation starships, like the Enterprise. (TNG: "Suddenly Human")
According to the Star Trek Encyclopedia (3rd ed., p. 190), this weapon has a basis in 20th century technology, developed as part of the Strategic Defense Initiative.