One of the criticisms thrown Thor: Love and Thunder's way is how underutilized Gorr The God Butcher was. By extension, that also meant one of the darkest weapons in Marvel history didn't get as much love as it should have. Although it is enormously powerful, the Necrosword is just one of many dark artifacts in Marvel's history.
From dark mirrors that grant those who see it corrupting power to blades that drain the blood of their foes, there's no denying how dangerous Marvel's arsenals can be. Some don't even have to be magical to have dark implications. They just have to be a final resort.
The Darkhold
The Darkhold is a tome of dark magic written down by the Elder God Chthon, an ancient god of evil. Scarlet Witch is its greatest user, and quite unfortunately for everyone else, it did succeed in corrupting her for a time. The Darkhold's power lies less in any inherent magical power, and more in its contents.
In the hands of great magicians, the tome is a terrifying weapon. A few examples of the spells in its pages are summoning Chthon and other elder gods, conjuring Darkforce, dominating the will of others, and the Montesi Formula, a spell that annihilates vampires. The Darkhold, to ensure its corrupting nature, is intelligible to any literate reader.
The Ka Stone
The Ka Stone, or the Spirit Stone, is a powerful gem that granted its user godlike power. With it, they could manipulate energy, read minds, fly, gain incredible physical upgrades, and become immortal. However, there is one big catch: their personality is overridden by that of Anath-Na Mut, otherwise known as the Sphinx.
The Spirit Stone also curses their bearer with an obsession for the Sphinx. Even exposure to it for prolonged periods of time makes people want its power. It's enough of a threat that Galactus himself decided to punish Sphinx by making him relive his life, but without the Ka Stone.
Crimson Gem of Cyttorak
The Crimson Gem of Cyttorak is an ancient jewel fueled by the magic of the demonic god Cyttorak. With its power, the bearer turns into the manifestation of Cyttorak's will, the Juggernaut. The Juggernaut has incredible strength and stamina. They also no longer need to sleep, eat, drink or even breathe.
Not only that, the bearer gains the Juggernaut armor, a nigh impenetrable suit of armor that he can summon at will. All of this sounds like a great deal, but it comes at a cost. The avatar of Cyttorak is cursed with an overwhelming desire to destroy everything around it. Such is Cyttorak's will, and so, the Juggernaut can't use their powers in a way that would displease Cyttorak. Good or bad does not matter. Only destruction.
Bloodstones
The Blood Stones, or Bloodgems, were part of a meteorite that struck earth thousands of years ago. It was controlled by an extradimensional being known as Exo-Mind. Ulysses fought off Exo-Mind's champion, but in doing so, shattered the meteorite, with one embedding itself in his chest. Now, the Bloodgem gives those who are merged with it great power.
However, there is a catch. The Bloodgem, upon its destruction, latched onto the emotion of Ulysses at the time. The meteorite's explosion had killed his entire tribe. In his rage, he vowed vengeance against Exo-Mind's champion, and so the Bloodstone would grow in power the more vengeful its host was. Unfortunately, that negativity also meant it was susceptible to being cursed, as Elsa Bloodstone and Daken found out.
Twilight Sword
The Twilight Sword is Surtur's weapon, and the very same sword he uses to wage war against Odin to astonishing effect. Its power is so great that it is said it could "set the universe aflame." Fittingly, it also holds the title of the Sword of Doom. Despite its dark uses, it actually holds the same enchantment as Mjolnir.
Both weapons are only useable by the "worthy". Once, it refused to be wielded by a severely weakened Surtur, and Beta Ray Bill became its new owner. Despite its history of blood and genocide, Beta Ray Bill used it to liberate others from Surtur's will. A powerful blade, but with a dark history, only respecting strength above all.
The Black Vortex
The Black Vortex was created by a Celestial to explore the concept of "desire". In doing so, the mirror became a portal through which the viewer could see their full potential realized. The Black Vortex amplifies the powers of those who accept its offer to absurd degrees. Kitty Pryde once used its power to save an entire planet from destruction.
However, such power comes at a cost. The Black Vortex elevates people to their full cosmic potential, but at the cost of their souls. Prolonged usage could cause insanity, uncontrolled fluctuations in power, and an obsession to be "more". At the moment it lies at the center of a star to prevent the wrong people from looking into it.
Staff of One
The Staff of One is a magical artifact with power so great that it makes even Dormammu tremble. The Staff of One isn't actually "of One" and more like "is One". As in, the Staff itself is actually the magician One, who was turned into a weapon by Nico Minoru's ancestor Tokiko after he defeated One in a grand war.
The Staff of One utilizes blood magic and requires a small offering of blood with each use. The more blood is used to fuel its magic, the more potent the spell becomes. Any spell can be cast with a command. For instance, Nico Minoru once wished that she had Vision on retainer, as his encyclopedic knowledge of all languages would likely make her unstoppable. However, each phrase can only be used once.
Ebony Blade
The Ebony Blade is a mystical sword forged by Merlin and Sir Percy of the Round Table. It is one of a complete "set" of weapons made from a meteorite called Starstone, and is the most iconic of the bunch, thanks to Black Knight's use of it. It is Excalibur's dark counterpart and bears with it a curse to its wielder.
Wielders of the Ebony Blade find their negative emotions amplified, at the cost of great power. Knull's comments on the blade imply it might be similar to the All-Black. Unlike the All-Black, however, the sword is much more magical in nature. At its most powerful, it drinks blood and drains people's souls.
Ultimate Nullifier
The Ultimate Nullifier is terrifying because of how absurdly powerful it is. Many weapons claim to be the most powerful, but there's no doubt that the Ultimate Nullifier is the most powerful weapon in the Marvel Multiverse in terms of sheer power. Its function is simple and devastating: it erases its target from existence.
Note that the target here is not limited to physical concepts. The Ultimate Nullifier's power is limited by the wielder's will and imagination. Should they lack either, the Nullifier will erase them instead. In the hands of Galactus or Reed Richards however, the weapon can destroy the entire universe. Thankfully, neither are keen on testing out the theory again.
The All-Black
While there is no doubt that the Ultimate Nullifier is the strongest weapon, the All-Black is the darkest weapon of the Marvel multiverse. In truth, the All-Black is not a mere weapon, it is actually the first ever symbiote created by Knull, billions of years ago. It was honed into a weapon, and since then, has corrupted each and every one of its wielders.
The Necrosword feeds off of negative emotions such as despair and hatred, then amplifies their host's powers to godlike levels. This is why Gorr is such an effective god killer. With every god he kills, his hatred grows, and so does his power. When it takes control of a being like Galactus, even worlds are but mere playthings. The symbiote is sentient, and unfortunately, its only desire is cosmic genocide.