Like most evil empires in fantasy stories, the Elven Empire is three things primarily- as unquestionably evil as DeBeers, intimidating and overpowering as a parent interest group, and as vaguely defined and poorly labeled as an old VHS tape found in a box in your basement.
Although it is clearly an empire, there isn’t much information about its ruling classes, political structure, or history. As far as anyone can tell, all it took was one disgruntled nut to make a single push, and the entire elven community snowballed into a massive force of genocide and domination the likes of which have never been seen.
It is heavily speculated that much of the elven community had been waiting in secret to execute just such a coup for years now.
The Empire is composed of nothing but elves, with the exceptions of those in Mialee’s inner circle who fall outside the race requirement, such as Santa Claus and Amelie Poulain. It operates primarily from the Blanded Plains of Greyhawk, although there are strong Imperial forces situated in both the Endless Expanse of Faeruns and in Aldea (where there are enough perfectly manicured forest gardens and beautiful fairytale castles to satisfy the elves’ racial longing for nanciness).