General Zelgius lives a dual life: under his true identity, he is revered as the most esteemed and honorable general of the Begnion Empire and is a vassal of Prime Minister Sephiran; and as the Black Knight, he is a mysterious servant of the Kingdom of Daein whose true identity is unknown even to its king, Ashnard. Both identities are regarded as immensely strong and talented warriors, and as the Black Knight, he also wields a variety of mysterious magical artifacts, including blessed armor that repels weapons that are not themselves blessed; warp powder that allows him to instantly travel anywhere, at the cost of exhausting him; and the divine sword Alondite.
The Black Knight first appears early in Path of Radiance at the outbreak of the Mad King's War, when he challenges Ike's father and his own former mentor, Greil, to a duel in order to retrieve Lehran's Medallion. He kills the aging and crippled Greil far more easily than he expected, right in front of Ike, and flees the scene when Caineghis interrupts them; he leaves behind Ragnell, the counterpart to Alondite, which the Black Knight had unsuccessfully attempted to goad Greil into using. From this point, Ike's quest for vengeance against the Black Knight is one of his biggest motivations throughout the war, but whenever they meet, Ike is unable to harm him at all with his ordinary swords. Near the end of the game, Ike and the Black Knight face each other at Nados Castle, with Ike now wielding Ragnell; Ike emerges triumphant, and as he escapes the castle crumbles behind him, leaving him to presume that the Black Knight has died.
Three years later, in Radiant Dawn, the presumed-to-be-dead Black Knight resurfaces in the fight for Daein's liberation from Begnion as an ally to the Daein freedom fighter Micaiah. After saving her from an ambush by hostile forces, he comes to her aid in the final battle to free Daein. When the war between Begnion and the Laguz Alliance coalition breaks out, Zelgius fights on both sides: as the Black Knight, he continues to serve and protect Micaiah; and as himself, he reluctantly leads the Begnion charge but is embroiled in conflict and infighting with Begnion politicians, and when the Begnion forces experience a schism over the conflict, he sides with Empress Sanaki, Sephiran, and the Laguz Alliance against the politicians. He has become fixated on Ike - who now serves on the side of the Laguz Alliance - as a rival: in the Nados duel, Ike had mentioned that Greil had crippled himself, robbing Zelgius of his desire to face Greil as he had been in his prime, so the Black Knight chose to let Ike win in the hope that he would grow stronger and provide him a future challenge worthy of the prime Greil.
Zelgius survives the judgement of the awakened goddesses Ashera and Yune, and as the Black Knight appears before Micaiah and unsuccessfully tries to convince her to stop fighting so that he can protect her. In the finale of the game, the Black Knight confronts Ike, Micaiah, and their party in the Tower of Guidance, standing in the way of their climb to its top to face Ashera, and reveals his true identity. Ike and Zelgius duel for the final time: Ike wins, mortally wounding him, and Zelgius dies content that his wish to face Greil in his prime has, by proxy in Ike, been fulfilled. Before and after the duel, Ike and Micaiah see visions that reveal Zelgius's past: he was born in Daein as a Branded, and while that mixed beorc/laguz ancestry gave him both incredible strength and long life, it also made him an outcast among both species. Sephiran - true identity Lehran, an ancient laguz - had taken him in and made him a key player in his plans, winning Zelgius's absolute loyalty with the promise that he would no longer need to be an outcast.
In combat, the Black Knight is one of the most powerful units in either game. He belongs to the armored unit archetype, but has higher stats and unique traits compared to regular armored units. He has immense strength and defenses, and in Path of Radiance, he has a blessed armor effect which makes him immune to all weapons at the player's disposal up to that point except Ragnell (this effect has faded by the time of Radiant Dawn). Even when the blessing effect is being bypassed, however, he can still power through opposing attacks and sustain relatively little damage. Like Ragnell, his sword Alondite has the bonus ability to attack over long distances by slinging shockwaves. In addition to his recurring boss appearances, Radiant Dawn gives the player the opportunity to control him in three chapters played from Micaiah's perspective, in which he is by far the strongest unit at their disposal. Additionally, after his final defeat in Radiant Dawn, Alondite is given to the player's army and can be used by any advanced sword-wielding ally, with all of the same strength and abilities as before.
The Black Knight trophy is exclusive to Super Smash Bros. for Wii U.
As a costume
In Smash Tour
In Smash Tour, the Black Knight is a red item. A player using the Black Knight at the beginning of a battle will cause their smash attacks to occasionally have one-hit KO power at random for that battle.
As an Assist Trophy
The Black Knight appears as an Assist Trophy in Ultimate. He moves across the stage slowly, attacking any nearby opponents with his sword, Alondite. His attacks are extremely powerful, and they trigger Special Zoom for most of the time. While he can be attacked and defeated, his HP is relatively high, he can't be grabbed, and he does not flinch from most attacks. Training Mode describes his Assist Trophy: Slowly approaches, and then executes several extremely powerful slashing attacks.
In the English version, the Black Knight has three quotes that have him say English words.
As a costume
The Black Knight Mii costume returns, and is purchasable from the Shop.