No peace, only suffering.
No knowledge, only instinct.
No serenity, only passion.
No harmony, only chaos.
No life, only death.
―The Dark Jedi Code, as written by the first Bogan
Dark Jedi are force-sensitives, frequently former Jedi, who chose to follow the darkside. Although "Dark Jedi" originally referred to a Jedi who had fallen to the dark side, it could also refer to uninitiated Force-sensitives who received no Jedi training but began their careers under another Dark Jedi. Others were simply dark-side users who did not follow the teachings of the Sith or other dark side organizations.
The term Dark Jedi is a collective and vague name for dark side practitioners (be they fallen or rogue Jedi) that use the force and a lightsaber, but are not members of a certain organization; whereas the term Sith refers to a definite heritage or ideology. Some Dark Jedi are allied to the Sith or other organizations, while others are loners. Because they are notoriously dangerous, unstable, and treacherous, Dark Jedi seldom formed well-organized groups on their own in order to expand their following or reinforce their strength.
Dark Jedi have no code. They do not follow set guidelines and buck against almost all rules. Those that follow Sith Masters are often doing so in hopes of becoming Sith themselves.
To become a Dark Jedi is simple, be a dark-sider. To progress along its path you simply do evil things for selfish gain. You will not have access to the disciplines of the Sith.
Dark-siders can change easily enough. A Dark Jedi can become a Sith, find spirituality and purpose and a teacher. They can grow beyond the normal limits. Likewise a Sith can become a Dark Jedi. They forsake the code and do evil just to do evil.
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To ascend in rank as a Dark Jedi we need to see how you play. If you are to lawful, following rules and tenants (similar to the Sith) then you are not a Dark Jedi and should go through testing as a Sith (which is more complicated).