These vessels were created in specialist shipyards in the hope that they would never be used. They were made in case the empire was to ever fall that the Romulan people would have one last act against their enemies. Any captain that piloted such a vessel would face certain death in its use which is why the Tal Shiar was known to use unorthodox 'conditioning' techniques to ensure that such captains did not falter when the time came to activate the weapon. Such actions were essential in making the captains of the Phoenix-class vessels loyal and commited in the cause of the Star Empire.
The weapon employed by the Phoenix-class was a rift maker that would create an expanding tear within the Space-time continuum that would destroy all in its path. However, as a result of the creation of the rift, the vessel would be destroyed and its crew would die.