The alter-ego of Begnion's Military Commander, Zelgius, he was born a Branded (half-beorc and half-laguz), and ages more slowly due to his laguz blood. He wears his famous armour to disguise this fact from his fellow Daeins, who despise both the laguz and the branded.
He was trained by General Gawain (also known as Greil), whom he has always wanted to best in a duel to prove his strength. Even after Gawain left Daein, the Black Knight remained in service of King Ashnard the Mad. He is eventually approached by Sephiran, the Prime Minister of the Begnion Empire. Zelgius becomes commander of the Begnion Central army while the Black Knight remains the strongest of Daein's Four Riders. He uses warp powder to get from place to place. Although he has sworn loyalty to Daein, by the time the series begins, he is loyal only to Sephiran.
Path of Radiance
By chance, he meets his old teacher in Gallia shortly following the Daeins' invasion of Crimea. The two partake in a duel in which he offers Greil the sacred sword, Ragnell. Greil does not take the sword, because he obtained an injury to his wrist causing him to no longer be able to wield swords. The Black Knight is unaware of this and defeats him in the duel anyway. After killing Greil, with Greil's son, Ike, watching, he is not satisfied. Had his old mentor's skill deminished over the years?
Ike swears to kill the Black Knight for what he has done. They meet many times throughout Path of Radiance. General Zelgius leads Begion troops to capture Daein and Ike even thinks he is "trustworthy enough". Of course, he did not know that these two are one in the same. Only after Ike obtains Ragnell do the two finaly duel at Naddos Castle. At this time, Ike explains Greil's injury to the Black Knight's distaste. After Ike has defeated him, the castle collapses on top of him.
He didn't actually die. He appears three years later in Daein during its occupation by Begnion. He assists the Dawn Brigade in overthrowing Begnion's rule of their country. He spends the rest of the game aiding Michiah as the Black Knight and leading the Begnion army as Zelgius. A laguz named Ranulf discovers the link between the two and divulges this information to Ike.
After the Goddess wakes, he waits in the Tower of the Goddess until Ike appears. The two duel and Ike truly deaftes him. As he lay dying, he tells Ike that he saw Gawain in his swordmanship and is at peace knowing he has truly fought his mentor once more.
The Black Knight drives much more of the plot than he is tradiationally given credit for. He does a lot of work behind the scenes to fufill Sephiran's designs.
The Black Knight bumps into Sephiran by chance while fighting Ranulf. Sephiran tells him not to pursue Ranulf or the fleeing Greil Mercenaries and the Black Knight listens. This is the first time the player knows of a link between the two of them.
The Black Knight and Sephiran once stumbled upon the site of a massacre. A town was destoryed by Greil's power alone. Greil's children, Ike and Mist, saw him kill their mother in a confused rage. Sephiran erased the childrens' memory of this and together they hid Greil from suspicion.
General Zelgius refuses to kill the unarmed Queen Elincia and is almost executed by Begnion Senator Valtome for this act of chivalry.
Zelgius has a loyal follower named Levail who is very much to Zelgius as Zelgius is to Sephiran.