Cartagia was Emperor of the Centauri and a dangerous ally of the Shadows on the Babylon 5 TV series. He's portrayed by Robert Krimmer.
Crowned Emperor of the Centauri and Madness
When installed as emperor in 2259 by a group of Centauri politicians led by Lord Refa and Londo Mollari, he acted as, essentially, a powerless figurehead; however, he gradually amassed near absolute power. The scale of his self-aggrandizement and madness were exposed in early 2261, near the end of the Shadow War, when he willingly brought Centauri Prime to the brink of annihilation by the Vorlons in an effort to secure his place among the gods.
After defeating the Narn in a series of quick strikes aided by the Shadows and illegal mass drivers, Cartagia's government undertook military campaigns against other races bordering Centauri space.
In early 2261, Cartagia had Babylon 5 Ambassador Londo Mollari recalled to Centauri Prime, and appointed him as advisor on planetary security. Londo learned from Morden that Cartagia had allowed the Shadows, reeling from the events on Z'ha'dum, to establish a base on the Centauri homeworld. Mollari expressed his concerns to a member of the royal court, who explained that though the Centaurum had tried to oppose Cartagia, all of his opposition had since disappeared, with the rumor being that Cartagia had hidden their heads in a secret room, where he spoke with them on a regular basis. Mollari later discovered this rumor to be true.
It was also around this time that Londo learned from Sheridan that the Vorlons were employing a new strategy against the Shadows: utterly destroying any planet with even the slightest taint of Shadow influence. When Londo informed Cartagia of this, he resolutely refused to evict the Shadows from Centauri Prime, explaining that they had agreed to reward his assistance by turning him into a deity. Cartagia then explained that the Vorlons would help his plans for deification by turning Centauri Prime into a massive inauguration pyre.
Conspiracy against the Mad Emperor
During these events, G'Kar was captured and brought to Centauri Prime. The emperor had initially said that he was making a gift of G'Kar to Londo. Presumably, so that Londo could torture and punish him. Outwardly, Londo thanked the emperor. In truth however, Londo was repulsed by the idea. In fact, Londo did his best to shield G'Kar from harm. As time passed, Cartagia grew to hate G'Kar's strong spirit and sought to brake him. He wanted one scream from the proud Narn. To this end, Cartagia had G'Kar mercilessly tortured; all the while G'Kar refused to scream or give into the pain. Londo advised him to give in for fear that he would eventually be killed in an effort to give Cartagia the scream he wanted. As part of the torture Cartagia proceeded to have G'Kar's eye plucked out because he didn't like the way G'Kar looked at him. After giving Cartagia the scream he wanted, if only to save his own life Londo met with G'Kar in his cell. The two proceeded to come to an agreement. In return for G'Kar's assistance in his plot to assassinate Cartagia, Londo would end the Centauri occupation of the Narn. As part of this ruse, Mollari convinced Cartagia to take G'Kar to Narn and execute him there, thus allowing Mollari to draw the Emperor away from the royal court, leaving him more vulnerable.
Death on the Narn Homeworld
When they arrived on Narn, Cartagia planned to have G'Kar killed in front of local Narns to break their spirits. However, Mollari had arranged for the chains on G'Kar's restraints to be weakened to distract Cartagia long enough for Londo to act. They would then secretly inject a poison into the Emperor which would cause his cardiovascular system to shut down nearly instantly.
G'Kar managed to break free and begin fighting with the guards, and in the following commotion Mollari escorted Cartagia away from the fight and prepared to inject the poison into him. Cartagia went into a fit of rage about how this was not part of his destiny and, reacting to being told to quiet down by Mollari, turned and punched him, knocking the device away. Cartagia grabbed Mollari in a stranglehold, ranting about how Mollari deserved to burn with the rest of the Centauri for failing to see his greatness. As Cartagia turned to walk away, he was injected with the poison by Vir, who had picked up the device while Londo and Cartagia struggled.
Shortly after this, the Imperial guards found Cartagia, supported by Mollari and Vir. Mollari explained that the Emperor had collapsed, and that he believed the Emperor's hearts had failed. Mollari kept his promise to G'Kar by explaining to the court that the Centauri's dealings with the Narn have now resulted in the deaths of two Emperors, and that Cartagia would interpret this as a sign from the gods to leave.
In 2271, John Sheridan met Cartagia's son Dius Vintari, second in line for the throne behind Vir. The reason he's second in line is because his father had made a lot of enemies in the republic and the government does not trust his son, for they fear he is just as mad as his father and is often targets of assassinations. Galen showed both John and Dius visions, to John, visions of the Centauri attacking Earth and to Dius, visions of meeting John Sheridan in an Earth fighter. John decides to take Dius in to not only keep him safe from assassination attempts, but to raise him to make sure he does not follow in his father's dark legacy.