Run for long enough and you're going to get some plot idea or another that revolves around some artifact of great power. In RPGs these artifacts quite often take on the aspect of a god's weapon. Perhaps it is a knock off of mjolnir - Thor's hammer - or gae bulg - Cuculain's spear - but somewhere, somehow, for some reason, you're likely going to want to have god weapons of some sort in your game. As I am currently in a place in my L5R game where I am looking at doing just that, why don't we talk about it?
If You Don't Want A PC To Have It....
don't put it in your game. Really, this is the simplest thing in the world and you'd be amazed at how often GMs get frazzed out by it. If you put a weapon, or some other item, in the game, then expect for that item to end up in the hands of a PC somewhere and at sometime. Why? Well, odds are because the PC will take it. If it is the "Great Sword of World Rending" that one of your antagonists wield, and the sword doesn't innately do horrible/evil things to its wielder, then is it all that surprising that someone in the party will grab it as a trophy?
Also, if you do put it in your game and when it does end up in your player's hands, don't balk. Just run with it. If the item is "too powerful" for someone of that level, then show the consequences of not respecting the power - assuming the PC abuses it. Weapons of power like that often come with people seeking them too, and that can be another plot. However, don't just take it away because you don't want a player to have it. By putting something in the game world you are directly inviting a player to take it on. Why else would it be in the game?
Everything Has Rules
I don't mean the mechanical effects that you may or may not need to represent what the weapon does. However, artifacts are often governed by rules. The more powerful the item, the more powerful the rules are. Mjolnir - in the Marvel movie universe - is an unparalleled tool of both destruction and creation, but because of this it is also very choosy in who it lets wield it. Many of the most powerful magical effects in D&D only key against certain enemies (i.e. holy vs. the infernal/undead.) The point being, have these rules in place before the weapon goes out into the world and you have the means to keep it under control if that is what you need. You also then get a metric to judge the PCs use of it when they get it. I mean, how long will Grithane the +5 intelligent elven defender against green dragons stay happy if it isn't fighting greed dragons?
People Tend To Notice
One of the fun things about having powerful weapons in your game is the fact that they give you an easy way to engage the character. People, especially people of a similar profession to the PC, will probably notice the high quality piece of equipment. This can lead to fun conversations, bragging sessions, challenges, and even its own adventure threads as the world reacts to the player having that weapon. Obviously the adventure possibility keys off of how powerful/prominent the item is, but you can still have fun with it.
Have A Story
Weapons of power and high ability should have a story. Something that grounds them in the world and recounts some of the past deeds that they have been used to perform. Sure, everyone loves the +5 holy long sword that they found in the temple, but the Holy Avenger that slew the great Lich King Balamut is a bit cooler, no? The stories don't even have to be long and detailed, just some neat fact to help ground the story. (This hammer was held by the hero Tori and wielded through the fall of Meralathian during the Great Rune Wars!)
Special and Unique
Finally, for this post anyhow, each item should have a flavor and you want to do what you can to keep that flavor unique. Yes, you have some wiggle room with sister weapons and matched sets that are supposed to be against/with each other, but for the most part you want some uniqueness to each weapon. If nothing else this helps the players feel special. In an L5R game I am in one of the players currently has the hammer of an aspect of Osano Wo, and it is freaking awesome. However, it becomes less awesome if suddenly other characters in the game start getting hammers with near identical abilities. Then it is just another piece of equipment. Sure, everyone could have a god weapon, but they should be different. Fire Thunder's Hammer, the Katana of the Wind, Akodo's Yari, and Shosuro's Blade could all be present in the game, all be awesome, and still give each character their own unique flavoring based on how they got their god weapon and just what it did. Be creative, you know you want to.
Do you have any thoughts or anecdotes about powerful weapons in your games? Sound off in the comments!