Hmm. . .
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How about a recurring legendary weapon from multiple of my campaigns? (Like Excalibur/Excalipoor or Masamune from Final Fantasy, there are a select few "legendary" things that are recurring between my universes, but only in a few select cases actually have any impact on the storyline.)
Name: Fate
Origin: In its original universe, it was crafted by a master spellcaster/smith with a penchant for justice. It varies in other universes.
How it ties in to the story: In it's original context, the weapon was forged with the sole purpose of being the ultimate weapon to execute justice with - but the weapon also served a massive metaphorical significance. In effect, it represented its wielder's grasp on reality, his grasp on justice, and his grasp on inevitability. The weapon, in time, is part of his corruption (or dive into insanity) - not through any malevolent force, but through a perversion of ego.
What it does: Fate is a curved-edge sword (read: Iato-style katana) that only functions when it is accompanied with its sheath. The blade, which looks like glass, by itself is literally incapable of injuring anything - the blade passes through all material matter as though it were not there. However, a reaction occurs if the wielder casts magic upon the blade while it is within the sheath (this is mostly done using the runes already engraved upon the sheath). The various spells enchant the blade, and after it is drawn, the blade will be able to place curses upon all things that it passes through. Nothing actually happens, however - until the blade is sheathed again. When the tsuba (the handle-guard) of the sword makes contact with the koiguchi (mouth of the scabbard), it instantly triggers all of those "curses", causing irreversible damage to all those the blade passed through regardless of the subject's actual defenses, as though it were an unstoppable magical force, complete with explosions based on the individual spells cast. Seeing as the blade passes through all things - including any enemy attempts at blocking - this weapon easily surpasses most any foe, assuming the wielder is skilled enough to not need to block them-self.
Weaknesses: As previously mentioned, the blade is incapable of blocking, and the sword, when not enchanted with its temporary magical effects, is completely incapable of dealing any damage whatsoever. It takes a master to wield this blade properly, even among the few who have the multiple talents required to wield such a thing.
Strengths: When used properly, this weapon is an unstoppable force of nature - hence its name.
Risks: The weapon itself possesses no curse towards its user - it is merely incredibly difficult to master. That said, between the three universes where it has popped up, it has been indirectly responsible for tragedy in all three cases - all involving the hubris of ego, and the loss of loved ones. No, this is not even by my plotting - it honestly just seems to happen naturally.
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Now for something a little bit more universe specific. Only one universe got this one.
Name: The Chaon Engine (Pronounced Kay-On)
Origin: Developed by scientists and engineers on Terra-formed Mars, trying to create the ultimate energy-provider, in an effort to bring peace to a long-running war across the solar system.
How it ties in to the story: It's the mcguffin of the story - anyone that finds out about it wants it for their own personal goals. In the end, the Final Big-Bad used it to power the ultimate battle-mech, simply referred to as "The Beast". (The Beast wielded a giant sword that cut into the fabric of reality, basically creating wormholes wherever it went.) The Chaon Engine serves as a metaphorical "Pandora's Box" in the story - the story's main driving theme was "Power beyond Humanity", in the sense that there is a point where one may have so much power that they are no longer human.
What it does: The Chaon Engine is an impossible technology. It is intentionally never fully explained, but the concept itself is rather simple: It is an engine that, within a anti-gravity cell, contains a small black hole. This black hole is manipulated using gravity-based generators, essentially creating infinite energy. There is a reason only one was ever made, and it is a slightly flawed model at that.
Weaknesses: Infinite energy = infinite potential for harm. The Chaon Engine was, in the end, utilized for a super-killing machine, and was effectively responsible for the destruction of an entire nation, as well as almost destroying the entirety of Mars. In a sequel to that storyline, the engine is also responsible for nearly collapsing the sun itself. It's too dangerous to possess.
Strengths: It is an infinite energy source - if protected, it could basically end all international conflict. If your intentions are less than pure, it can easily be used to create powerful weaponry.
Risks: Okay, I'm not repeating everything I just explained. tl;dr: It's too dangerous to possess, or allow to exist for that matter.
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Finally, a third example: This one is also universe specific.
Name: "Memory-Spray" Weaponized Anesthetic
Origin: Developed by a team of scientists working for an Illuminati-esque organization, the chemical device was devised for the purpose of cleaning up "messes". Out-of-character, it is obviously inspired by Men In Black.
How it ties in to the story: It is a gadget/material used by said organization, and does not play a specific over-arching role. Yet.
What it does: Depending on the purity-grade of the chemicals involved, canisters of this spray can be used to wipe the memory of victims who inhale or otherwise intake the chemical. A single dose of the most common purity can wipe someone's memory of the last 5 hours, but the strongest stuff is capable of wiping a human's memory entirely. The material is only entrusted to specifically licensed members of the organization, but the material is perfectly capable of being lost in the field.
Weaknesses: If lost, may be turned on user. If damaged, earlier models of the spray canister would explode the material (no longer possible). The spray does not necessarily work on super-humans or monsters (case-by-case basis).
Strengths: Nothing is better at making a bystander forget they just saw a rampaging hydra in the middle of the street.
Risks: See weaknesses - it is just as dangerous for its user, so it is usually recommended to wear a gas-mask before using. Those that know of this technology have already attempted to use it for international-espionage - there may be some brainwashed international leaders.
