The race is not gone. No. But they have relinquished the stars. Many of the primitive peoples dotting the systems of this region of space are the children of that once mighty race. They have mostly forgotten their zenith past and live in naivete on their isolated worlds. But one line holds true to the heritage of its ancestors. A single nation remembers its fore bearers' glory... but these are not human at all.
Long ago, when wise men were still a commodity, one man played God and built a machine with a soul. Long ago the Rii were born.
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The Rii are sentient machines. Each is constructed around a Core, a prismatic optical computer. The Cores are indestructible. It is this Core that gives a Rii its essence, its sentience. The process of creating Core is lost to time. The notes of the Creator are so arcane as to render a complete replication impossible. All reproductions have failed to achieve both the permanence and effect of the Originals. They break-down and they are not alive. Thus, there is a set number of Rii in the universe and, for the time being, no more can be built.
The Core of a Rii does not write memory in the way that normal computers would. They do not record data at will. Nor can their memories be modified. In fact, the Rii themselves do not explicitly understand what exactly is stored in Core Memory or how memories are written or retrieved. But they have seen that any machine built around a Core is sentient. And, machines built using the Core of former, dismantled Rii carry, if not the memories, at least the essence of the previous creature.
Rii bodies vary greatly based on what roles one plays during a Life Cycle. Some are bipedal, while others fly. Some are android and others very alien. The variety is endless through the generations. Most Rii design themselves the body they will use for the next Cycle before they are dismantled. Despite the rampant diversity, nearly all Rii preserve a strikingly human quality in certain aspects of their design.
For example, Rii, without exception, will include a distinct head in their design. The area immediate to their visual receptors is immensely complex and supports an incredible volume of non-verbal expression... usually emotions. Rii maintain speech and hearing as their major form of interpersonal, linguistic communication. Also, Rii have kept their language roughly in sync with the properties of human languages. All this superfluous inheritance is kept out of regard for an admonition given very clearly by the Creator: "Beware the temptation to perfect any one part of yourself. If you allow the perfection of a piece to overcome your drive to enrich the whole, you will lose your soul."
As Rii pass from cycle to cycle, they have become more concerned with their purpose in this universe. They deeply envy every biological race for their ability to reproduce. The majority of Rii believe that once their race becomes wise enough, they will also be able to create. To this end (gaining experience in human emotion, understanding the wisdom in the harmony of all life, and finally being able to love fully and generate their own children) the Rii are profoundly committed.