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My understanding is that the Demiurge existed from the beginning, along with the other Gods. Satan, Samael, the Demiurge and Yahweh are different names for the same being, as far as I recollect.
You could find the below on your own, but I was curious about Valentians, and thought I'd share:
It is in the system of Valentinus that the name Dmiourgos is used, which occurs nowhere in Irenaeus except in connexion with the Valentinian system; and we may reasonably conclude that it was Valentinus who adopted from Platonism the use of this word. When it is employed by other Gnostics it may be held either that it is not used in a technical sense, or that its use has been borrowed from Valentinus. But it is only the name that can be said to be specially Valentinian; the personage intended by it corresponds more or less closely with the Yaldabaoth of the Ophites, the great Archon of Basilides, the Elohim of Justinus, etc.
The Valentinian theory elaborates that from Achamoth (he káta sophía or lower wisdom) three kinds of substance take their origin, the spiritual (pneumatikoí), the animal (psychikoí) and the material (hylikoí). The Demiurge belongs to the second kind, as he was the offspring of a union of Achamoth with matter. And as Achamoth herself was only the daughter of Sophía the last of the thirty Aeons, the Demiurge was distant by many emanations from the Propatôr, or Supreme God.
The Demiurge in creating this world out of Chaos was unconsciously influenced for good by Jesus Soter; and the universe, to the surprise even of its Maker, became almost perfect. The Demiurge regretted even its slight imperfection, and as he thought himself the Supreme God, he attempted to remedy this by sending a Messiah. To this Messiah, however, was actually united Jesus the Saviour, Who redeemed men. These are either hylikoí, or pneumatikoí.
The first, or material men, will return to the grossness of matter and finally be consumed by fire; the second, or animal men, together with the Demiurge as their master, will enter a middle state, neither Pleroma nor hyle; the purely spiritual men will be completely freed from the influence of the Demiurge and together with the Saviour and Achamoth, his spouse, will enter the Pleroma divested of body (hyle) and soul (psyché). In this most common form of Gnosticism the Demiurge had an inferior though not intrinsically evil function in the universe as the head of the animal, or psychic world.
If you understand, things are just as they are; if you do not understand, things are just as they are.
~ Gensha, Zen Master