A powerful alien vessel belonging to a previous unencountered species in an aborted timeline just before Stardate 6171.8 in orbit around the rogue planetoid DT-262.
When the rogue planetoid had entered the aliens' native space its electromagnetic fields started to disrupt the aliens' mechanical and organic energies. The crew of the alien ship volunteered to investigate the new planet and found the USS Enterprise in orbit. The leadership assumed that the Starfleet vessel was somehow connected to the appearance of the planetoid which unusual properties now threatened their civilization and species and order an immediate attack on it.
Technologically this ship was superior in capabilities to a Constitutation class starship, able to out maneuvre the USS Enterprise and having superior engine speeds. It was also capable of generating a temporal warp field that could be used defensively, rendering the ship almost undetectable to Starfleet sensors, one would have to scan for temporal anomalies rather than a ship as the vessel did not fully “exist” in normal space. (when detected only the ship's outlines could be seen, the vessel itself being transparant), the temporal warp field was also part of a temporal phase weapon that could be projected at targets, acting as a shock wave in time and space rather than a conventional explosive weapon. According to chief engineer Montgomery Scott it seemed to operate in such a way as if “firing before it fires”. The temporal phase weapon did have its limits, it could only be projected a few thousand meters and consumed a lot of energy, making it impossible for the crew to use it repeatedly and use other ship systems to their fullest at the same time.
The temporal warp field was also somehow tied into the ship's propulsion system (possibly part of it), allowing the ship to jump in front and behind the Enterprise in time, putting enormous stress upon the starship that it was ripping the Enterprise apart. It would end up destroying a large part of the Enterprise's saucer section along with its bridge, killing most of the crew who had been on duty at the time including commander Spock.
Captain Kirk with a handful of volunteering security officers would eventually transport over to the alien vessel (either it had no shielding to prevent the use of a transporter or the Enterprise crew had been able to disable it) in an attempt to reach the vessel's command hub and stop its crew from continuing their assault on the Enterprise.
In the undetermined time after Captain Kirk and the security officers had transported onto the alien ship it had utterly destroyed the entire Enterprise, leaving Captain Kirk as the sole survivor after the aliens most likely had dealt with the security officers.
By altering the original course of planetoid DT-262 on orders of captain Pike, the crew altered history and as a result the Enterprise never encountered the alien vessel.