Fallen Empires are large and extremely powerful empires that have become old and stagnant with their societies grown inward over the ages. Some of these are created when the galaxy is generated at the start of every game. They have large and powerful fleets and is unwise to start a war with them unless your fleet is of equal power and should generally be left alone. If successful at conquering a Fallen Empire, it is possible to gain advanced technology from them. Only the AI can control Fallen Empires and they are not designed to be played by a human player.
Despite their size and power, they remain isolated and ignorant of the other younger empires outside of their borders unless provoked. Diplomacy is almost useless with them, however they will instantly contact you when you unwittingly enter their space. It is possible under certain circumstances that a Fallen Empire can 'reawaken' and could become a galactic threat. There are different kinds of Fallen Empires and react to certain events based on their personality. They are also only limited to one ethos (2 ethos points instead of the normal 3 points) which they are a fanatic in.
Technology and Ships
Fallen Empires use very advanced technology, they are even known to have ring worlds around stars. Their ships are equipped with 'Tachyon Lancers' and jump-drives. They use a unique ship and station art style which is only exclusive to Fallen Empires, which is to distinguish them from ships of normal empires. When one of their ships is destroyed it is likely to leave behind debris which a science ship can reverse-engineer and gain some Fallen Empire technology.