Situation: the campaign deals around the players lugging around a plot-important artifact that makes the user significantly more powerful than their peers (like the Rod of Seven Parts, a fragment of the Power Cosmic, the Anti-Cthulhu Amulet, a Bolter used by the Emperor himself at one point and still bears part of His essence, the Boots of Infinite Asskickings: just name something). Are there any ways I can have the other players not lag behind?
Sure, I could give them stuff that puts them on the same power level, but that might end up with negating the big deal that the artifact is. So how do I go about doing this?
Best Answer
There is a really awesome example of how to do this, right in the classic Fantasy canon: Frodo and the Ring. Frodo holds the One Ring, and it has what is - for the setting - an extremely powerful magical effect. And yet the Fellowship of the ring still feels powerful and important. I'm going to try and break down why that's the case, and hopefully this will help you in your campaign (I can't be super specific to your situation because you haven't really specified what your specific item does).
The wielder of the artifact must be protected
Frodo has the one ring, and he is in fact the only person who can bear it. But physically, he's probably the weakest member of the group. He contributes minimally in combat, and his stamina is less than legendary. Plus, every evil creature in the world wants to kill him and take his shiny. As a result, he spends a lot of time being carried and defended by other characters.
Frodo is still a potent and interesting character - a lot of the narrative is his struggle with the force within the ring - but he doesn't overshadow the other characters because the fact that he has the artifact gives them something to do, too. Without orcs and Nazgul chasing Frodo wherever he went, when would Legolas ever get to use his bow? Frodo's special trinket makes his companions more important, not less important.
To apply this to your campaign, make sure the artifact is not a magical swiss-army knife that can solve every problem. It can instantly win some conflicts, sure - Frodo gets out of a few jams with his invisibility - but it shouldn't explicitly replace any of the other roles in the party. If you haven't given the artifact to the party yet, and you still have time to design it, I suggest making it something that plays on or complements the abilities of the character holding it, rather than giving them new and unrelated abilities.
Using the artifact has consequences ... for everyone in the group
Not only does donning the One Ring sap Frodo's life force, it tells Sauron exactly where he is. And where he is, his friends are too. Suddenly a world of pain is dropping on their heads, and it's all his fault. If you implement a similar consequence in your game, it does two things: It creates story opportunities for the whole party every time the item gets used, and it gives the party an incentive to actively discourage the use of the item. This creates a really interesting dynamic which will definitely not be boring for anybody involved.
I ran a campaign a few years ago where this exact situation came up. The party had discovered a magic gem which contained an imprsoned god of chaos, and it naturally granted some crazy bonuses. They were spellcasting and concentration bonuses, so naturally the Sorcerer grabbed the gem for himself. Over the following weeks, the gem began to speak to him in his sleep, promising him greater bonuses if he were to cut out his own eye and replace it with the gem. He began to talk with the gem every night, discussing its offer and plotting to kill the rest of the party. The party got wind of this, and had to subdue him and destroy the gem, in a climactic battle atop an active volcano. Easily one of the most memorable sessions of the campaign, and it happened as a result of the introduction of a superpowered item.
TL;DR: Make sure the item has a specific effect with consequences for use, and use the item's presence to involve the rest of the party in a story where they all have roles.