The Black Knight is the name of two characters in the Marvel universe, the first being a villain, and the second a hero.
Nathan Garrett was a knowledgeable scientist and inventor and the first Black Knight. He was the descendant of the famous Black Knight of Arthurian legend, and sought to fulfill the role. But he was deemed unworthy to draw the original Black Knight's enchanted sword, the Ebony Blade, and thus could not become the successor.
Nathan then used his scientific knowledge to create highly-advanced, powerful armor and weaponry for himself, as well as genetically mutate a horse to growing wings as his mount. He became the criminal supervillain Black Knight, and was an enemy of Hank Pym aka Giant Man and the Wasp. He soon joined Baron Zemo's Masters of Evil to battle the Avengers. One of Black Knight's most common adversaries was Iron Man, and it was in a battle with him that Nathan was mortally wounded. On his deathbed, he regretted his criminal ways and passed his legacy on to his nephew, urging him to use the tech and power for good.
Dane Whitman inherited the castle that was previously owned by his uncle. He didn't know his uncle was the villain Black Knight until he found his notes and inventions. He eventually came upon the Ebony Blade. He passed its test and found out about its history and its curse. He decided to take the weapon and to be a hero, joining the Masters of Evil, just as his uncle did, only he had the intention to infiltrate them from within. He did this to aid the Avengers, but they didn't trust him until he assisted them against Kang the Conqueror.
The Black Knight wore a full-body suit of electrically-insulated steel-alloy chain mail that was bullet-proof. He also carried a lance that he equipped with many offensive devices including electrical blasts, heat rays, bolas cables, machines guns, buzz-saw disks, and forceful energy blasts.