I'm Making My Own Solo D&D 3.5 Campaign

[Bartmanhomer]

Excellent! That's a reasonable outline of the most important events; it sets you up for adventures centering around Herlon, and it gives you some tools to describe Harlon as the players adventure there.

When the players haven't been there for a while (or as you otherwise move history onwards), those issues get resolved- the devils are driven back, or not, or learn to coexist, but there's damage left behind; the high elves beat down the drow or vice versa, but it's stirred up underlying issues with the rest of the population, etc. To replace them, other news comes up- and note that not all news is bad news. Maybe the drow are now importing a valuable silk, and wealth is pouring in- if the players would import some looms, they could make a great deal of money.

From here, there are a few general directions you could go.
-You can focus in on detailing some of the individuals, districts, customs, or factions within Herlon. Do all high elves think the same way about the drow?
-You can zoom out on the larger kingdom of Drivor, and ask yourself some of the same questions as you did with Herlon. Is the devil invasion important enough for the king to care, or does he have other things on his mind?
-You can invent some neighboring cities of Herlon, and ask some of the same questions as you did with Herlon- but also, ask yourself how they relate with Herlon. Maybe one of them is the main source of the Drow that are immigrating to Herlon?

From there, you iterate; zooom in and out, but don't forget to draw connections between locations and groups as you go.

There's no king in Herlon. There's a mayor.