Thirdspace Aliens

The Thirdspace Aliens are a purely evil race from Babylon 5. Their malevolence is even greater than The Shadows. They are more ancient and more powerful than The Shadows and the Shadows enemy, The Vorlons. Where The Shadows were more about strength coming from conflict and chaos, they simply wanted to destroy all life. They only had one appearance in Babylon 5: Thirdspace made-for-TV film

The Vorlon Encounter
The Vorlons were a race that roamed the galaxy and were thought of by the younger races, like the Minbari and Humans, to be Angels or gods. This made the Vorlons arrogant and in their arrogance, they built a new type of jumpgate, one that wasn't capable of traveling though space, but dimensions. The Vorlons wanted to travel to what they called "The Well of Souls" only because they felt they outgrew this dimension and wanted to move on. They opened the gate and a race of pure malevolence came out of the gate and they tried to destroy the Vorlons. They nearly did because they were even more powerful than they are. They even telepathically manipulated some of the Vorlons to help them.

As the Vorlons who had their will was going to destroy the gate, the Vorlons who were under this old race's power hid the gate from them, but for the moment, the gate was sealed.

The Babylon 5 Encounter
Over a million years after their conflict with The Vorlons, the gate was found by a squadron of Starfury fighters led by Commander Susan Ivanova, Second in command of Babylon 5 and a member of the Army of Light, discovered the jumpgate within hyperspace after they had won a battle against raiders who were trying to raid supply ships. They brought it to Babylon 5 and once it was there, it was starting to effect people. Making them do things to speed up the gate's opening. It effected a telepath, Lyta Alexander, who had her abilities adjusted by the Vorlons, in a frightening way. Her Vorlon programing made her try to destroy the gate by any means necessary. Somehow the word of the gate went back to Earth and IPX (Interplanetary Expeditions) sent in a team to investigate it, provided they provide Babylon 5 with a supply of food while they work with the station. The leader of the IPX team, Elizabeth Trent, killed her friend and second in command, while under the alien's influence and allowed the gate to open.

Once the gate was open, an alien invasion fleet flew through it. They started to attack the station and the Whitestar Fleet as well as all other alien ships that was defending the station while inside Babylon 5, people who were under the influence of the aliens were fighting station security, leading to a massive riot. In the meantime, Babylon 5 commander, John Sheridan flew into the gate and set off a Nuclear bomb and destroyed the jumpgate, finishing the job the Vorlons started several thousand centuries ago.

It's not certain if they are locked out of the universe forever, but for now, the universe is beyond their reach, though Sheridan cited that if they return that no one would probably survive the invasion.