The time-traveling Kang the Conqueror is an iconic Marvel villain with a penchant for collecting trophies from his temporal adventures, but that makes him surprisingly vulnerable to the Black Knight's unusual form of time travel. Kang takes pride in beating powerful enemies and claiming something from them as a prize, from the sword of Conan to Thanos' Infinity Gauntlet. This obsession backfires with the Ebony Blade, the famed sword used by Dane Whitman, otherwise known as the Black Knight.
The Ebony Blade was created by Merlin as a dark counterpart to Excalibur, and given to Sir Percy, Dane's ancestor and the original Black Knight. The sword has its own slew of dark and mystical powers that can corrupt its wielder and draw out their bloodthirsty side, feeding on their corruption in exchange for increased power. However, the sword also has a time travel element that connects Black Knight and Kang.
In Avengers #71, by Roy Thomas, Sal Buscema, and Sam Grainger, a battle between Kang and the Grandmaster transports the Avengers to the year 4000 AD while Goliath is holding Dane's Ebony Blade. In the present, Dane focuses on his connection to the sword and - with the help of Sir Percy - travels to the distant future using the Blade as his anchor. There, he defeats Kang and his forces and rescues the Avengers. While Dane is ordinarily able to have the sword physically return to him like Thor's hammer Mjolnir, it turns out he's also able to transport himself to the sword, even over the gulf of time. This is a useful enough ability against Kang, but is even more relevant given the Conqueror's constant trophy hunting.
Kang the Conqueror has sought numerous artifacts throughout time, turning his base into a museum of all those he has defeated. Unlike most of the villains in Marvel's long-running continuity, Kang doesn't seek to use the prizes he collects, but he does see them as vital signifiers of his personal supremacy over all other life. This obsession is why the Ebony Blade presents a challenge for him. For as long as the sword is in his trophy collection, Kang runs the risk of Dane showing up at literally any given time to show the time traveler who's boss and take one of his prized possessions. It's a huge Achilles heel, but Kang still isn't the kind of person who could allow an object of power to exist outside his control. The real kicker is that even if Kang takes the sword from Black Knight personally, it should still be possible for an earlier or time-variant version of Dane to take advantage of this ability, meaning it's never safe for Kang to claim the Ebony Blade.
Kang might not want to admit it, but his obsession with powerful artifacts puts him in a never-ending nightmare - if only he had a way to break the bond between Dane and the Ebony Blade! Of course, Dane wasn't the first or last Black Knight, and still others like Black Panther have bonded with the weapon in the past. It seems that as long as Kang the Conqueror feels the need to claim the trophies of his conquests, he'll never be safe from some version of the Black Knight showing up on his doorstep.