Introducing a very old PC as an NPC questgiver? (Campaign shifting)

[MonkeySage]

I'm thinking of including an NPC in an upcoming campaign who, depending on the players' choices, may end up transforming my entire campaign setting overtime. My campaign is starting out in a standard medieval fantasy setting, like Forgotten Realms. This NPC is an inventor- he's a true neutral drow bard whose interests include uncovering lost ancient tech, and building new tech. The drow is a genius and a renaissance man of sorts, and given the opportunity he could trigger an early industrial revolution. But he can't do this without the help of the players, and I intend for their choices to be instrumental in how everything pans out- with different outcomes depending on whether or not they help him, how they help him, etc.

How should I handle this?

[Biggus]

I'm thinking of including an NPC in an upcoming campaign who, depending on the players' choices, may end up transforming my entire campaign setting overtime. My campaign is starting out in a standard medieval fantasy setting, like Forgotten Realms. This NPC is an inventor- he's a true neutral drow bard whose interests include uncovering lost ancient tech, and building new tech. The drow is a genius and a renaissance man of sorts, and given the opportunity he could trigger an early industrial revolution. But he can't do this without the help of the players, and I intend for their choices to be instrumental in how everything pans out- with different outcomes depending on whether or not they help him, how they help him, etc.

How should I handle this?

The danger with this sort of campaign - which, to be fair, it sounds like you're already aware of - is of it becoming the NPC's story, not the PC's story. Off the top of my head, I can think of two ways to avoid this: one, the NPC is slowly consumed by hubris and ultimately becomes a danger the PCs have to defeat, or two, at the same time as his inventions are changing the world, a dire threat of some kind is also growing, which the PCs need his inventions to stop (if they succeed, the industrial revolution and the transformation of the society then become part of the epilogue, the world they saved becoming a better world).