Plasma torpedo

From Memory Alpha, the free Star Trek reference.

The plasma torpedo weapon is an extremely powerful torpedo of high-energy plasma that is used by Romulan forces and the Cardassian Union. The plasma torpedo travels at multi-warp speeds, nearly fast enough to outrun a Constitution-class starship.

This type of high-energy plasma weapon was first encountered by Starfleet when a Romulan Bird-of-Prey destroyed several of the Federation's outposts near the Romulan Neutral Zone. The weapon drained the vessel of power, forcing it to decloak to fire, and limited the travel range of the vessel's fuel supply. Upon detonation, the torpedo would unleash an enveloping energy plasma, forcing an implosion in the target. The weapon had a limited range, the plasma effect would dissipate after a few minutes of faster-than-light travel. (TOS: "Balance of Terror")

Romulan plasma torpedoes give off high concentrations of trilithium isotopes. (DS9: "Image in the Sand")

During the 24th century, Kazon Predator-class warships were equipped with plasma torpedoes. (VOY: "Projections") Additionally, plasma torpedoes were standard armament aboard Cardassian orbital weapons platforms. (DS9: "Tears of the Prophets")

The Romulans assaulted the USS Enterprise in 2267 with plasma weapons, scoring several direct hits. (TOS: "The Deadly Years")

The plasma effect was shown in its later appearance as stock footage from "Balance of Terror", combined with footage weapons hitting Enterprise from other TOS Season 1 episodes. Since the Enterprise was hit several times by a weapon that had destroyed entire outposts in only a few firings, its possible the Romulans had adjusted the yield of the weapon for ship-to-ship combat, rather than asteroid bombardment as they were used in the original appearance. This may be because they were not necessarily trying to obliterate the Enterprise, the secrets they could learn from an intact but crippled ship would be far more valuable then debris. Also asteroids don't move, and starships do...less yield means less of a power drain on the firing ships on-board systems allowing the ship to retain some of its maneuverability and escape capability.

In 2375, the Romulan Star Empire constructed a hospital for its wounded on the Bajoran moon, Derna. In addition to the hospital, the Romulans also deployed 7,000 plasma torpedoes on Derna, for which Senator Cretak claimed were for defense purposes only. (DS9: "Image in the Sand") The Romulans were, however, unable to bring the weapons on line due to a Bajoran blockade of the moon and the threatened intervention by the Federation. (DS9: "Shadows and Symbols")