Powerful NPC allies are a problem in any setting. Their very presence can distort the game in various ways. Common threads when they appear are: why aren't they handling this problem? They may dispense with information, but why be coy? If they do help, why are the PCs even present? The easiest way to solve this is to simply remove them from the game - they don't exist, so the PCs are the movers and shakers in the raw power area.
In some settings, such as Earthdawn, this just isn't an option. Verisimilitude, and even the basic mechanics in Earthdawn's case, requires that the presence of more powerful NPCs in the world for a host of reasons. The most fundamental reason is training. For most of their careers, the PCs will receive their training from higher Circle adepts, meaning that there always needs to be someone bigger hanging out. Which is fine, but why aren't Master tier adepts hogging all of the glory?
This is the explanation that I have introduced to my current Earthdawn game. It draws on the metaphysical concepts already present in the setting, Karma, Patterns and Threads, and expands on them in new areas. When dealing with "big ideas" like this, it is generally my preference to build on elements that already exist within the setting/system. I feel it gives them significantly more authenticity, which is important when what you are really after is player buy-in. If you cannot get authenticity, always go for awesome, but that's for another day.
The essential idea is that as an adept grows more powerful, their connections to the world increase. They Weave Threads, their Legend grows, their Pattern infects objects important to them, and they bind it to items, people, places, concepts. As the adept gains more power, the greater the metaphysical "tangle" that surrounds them. It becomes impossible to track the full extent of their unintentional connections through their various associations. All of this is their Karma (their "deed" or "effect") and using their Karma sends tremors down those Threads, gently touching all of those connections.
When they are young and lower in Circle, there is nothing to worry about. Their Karma is little; the effect is minimal and doesn't spread beyond their grasp. As they grow more powerful, their deeds become significant, and the effects are similar. These emanate around them, touching other Patterns and Threads in ways the adept could never truly consider. Eventually, this Karma may rebound on itself, as the vibrations collide with other vibrations and cascade back to the adept. This is when the unintended consequences begin. When things start to go wrong, when the universe recoils on the adept, when their Karma is too great and they must begin to carefully consider their actions.
It starts in small ways, but will eventually envelope adepts that are not careful. Those that are reckless are lucky if it only affects them - few are so lucky. The practical result of this is that most high Circle adepts retire from the public eye. They remove themselves from the world, reluctant to return for fear of what their Karma may cause, what the tremors of their actions may result. Instead, they must select and train those to come after them. Advise, but never act. Even saying too much may have unintended consequences. For these reasons, they act carefully, always selective of what they do. To minimize their new connections, they may invest in a handful of promising adepts, but always cognizant of the danger their actions may hold to those young adepts.
To illustrate this, I give you a Tale of Two Weaponsmiths. Grommit and Firefly were two of the finest Weaponsmiths of their respective generations. They changed the world through their deeds, crafted the arms and armor for their friends and companions, and strode across Barsaive as heroes and legends. Their works were second to none, but it was not long before each of them noted the tremors and the causality of their actions.
When Grommit crafted a fantastic set of armor for the king of Cara Fahd, it was hailed as a masterpiece. It truly was a thing of beauty. It inspired glory, passion, pride in the king's people. However, to other Namegivers it inspired jealousy, fear and rage. The king of Landis requested a similar honor to be bestowed upon him - this incensed Grommit. Who was this human to make such requests of him? Grommit could not have predicted the fallout from that much later, as both Cara Fahd and Landis fell to each other in battle.
The Thread Items he crafted were the most insidious in the end, for their influence is still felt today. Grommit imbued them with all of the power he could, granting them an uncommon level of sentience. The lure of their power was seductive, but their long term effects were devastating. While all True Patterns have the power to subtly influence the world, which is why Thread Items are never truly lost, this influence was not subtle. They would twist events to their ends, bending those Woven to them. When their partners were broken, it wouldn't take long for another to find them. Part of this influence distorted any retelling or recording - words forgotten, recorded incorrectly, or happenstance would make the original passage impossible to read - the end result is the truth of these items was always forgotten, but their power and pedigree never were.
Most importantly, however, he would have the chance for one final creation. Completing his Heartblade, it would be his final gift to his daughter. He deeply feared any connection to him would bind her to his Karma. So he watched her from afar as he could, as much as he dared. Even when he finally did meet her, he still could not tell her, despite how he wanted to. How he ached to tell her how proud he was of her; he wanted her life to be her own, not an echo of his failures. It was then that he completed his Heartblade, binding all of his power into his life's work, this final creation that was the culmination of everything that he was. It was his final sacrifice, knowing that were he to live beyond his masterpiece's birth, the ripples would be tidal waves. That was his Dying Legacy to his only child that never knew him.
That is the thesis that I have introduced into my current Earthdawn campaign and it works nicely with/was created for one of the themes of that game. Specifically, that each new generation of adepts must rise to fix the mistakes of those that came before them. Beyond that, it (or something that borrows and/or adapts the ideas) can serve well for a number of other campaigns and settings where powerful NPCs need to be present without overshadowing the PCs. If you have any thoughts or questions, leave a comment. I would love to hear them and see if this can be developed in any other ways.