The Black Knight is a major antagonist and Ike's rival in Fire Emblem: Path of Radiance and Fire Emblem: Radiant Dawn. He is a mysterious knight who serves the kingdom of Daein, whose identity is not known even to the king he serves, Ashnard. Regarded by both his allies and his enemies as virtually invincible, in addition to his immense strength and talent as a swordsman, he wields goddess-blessed armor and his sword Alondite, the counterpart blade to Ike's ultimate weapon, Ragnell. Ike first encounters the Black Knight early in Path of Radiance, when he witnesses the death of his father, Greil, at the hands of the Black Knight in a duel. Ike's desire to defeat the Black Knight and avenge his father's death is one of his primary motivations as he fights in the war against Daein. Ike challenges the Black Knight multiple times throughout the game, but is incapable of harming him until he obtains the blessed sword Ragnell. Near the end of the game, Ike and the Black Knight have a final climactic duel in Nados Castle; Ike emerges triumphant, and as Ike escapes the castle crumbles behind him, leaving Ike to presume the Black Knight's demise.
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, and again when Daein is pitted against Ike, his mercenaries, and the Laguz he fights for. The Black Knight reveals to Ike that he allowed Ike to win in their previous duel; after he realized that Greil had crippled himself, he wished to battle Greil in his prime, so he let Ike live so that he may grow stronger and the two could battle again. After the awakening of the Goddesses Ashera and Yune, as Ike and his party ascend the Tower of Guidance to challenge Ashera, he meets the Black Knight again. The Black Knight's true identity is revealed to be Zelgius, the commander of Begnion's troops, and the two begin their final duel, which Ike wins. Before and after their duel, Ike and Micaiah see visions that reveal the Black Knight's past as a "Branded" (one with both human/Beorc and Laguz blood in their veins), which rendered him an unaging outcast among both Beorc and Laguz. He was eventually taken in by the ancient Laguz Lehran, who was masquerading as Begnion's Prime Minister, Sephiran. With the Black Knight's death, Alondite accepts Ike as its new owner (though any advanced sword-wielder may use it in his stead).
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. Robert Clotworthy and Junji Majima reprise their roles in both English and Japanese via recycled voice clips from Fire Emblem Heroes.
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.