First Sith Civil War

Some sources suggest that, rather than being destroyed by the Jedi Exile and the Lost Jedi, the Sith Lords who rose to power after the demise of Darth Malak destroyed themselves in a bloody civil war. According to these sources, the Exile did still have a hand in the death of the Sith, since he discovered that they were striking from Malachor V. However, the final blow to the Sith Order came at the hands of one another. This conflict was known as the Sith Civil War.

If the civil war explanation for the decline of the Sith of that period is true, then it provides yet another reason for Darth Bane to have established his Rule of Two, many millennia later. With only two Sith in the galaxy at any one time, it would prevent a disaster such as the Sith Civil War from happening ever again.

It should be noted that the same records which tell of this little-known conflict also indicate that either Darth Sion, Darth Traya, or Darth Nihilus actually survived the aftermath of the Jedi Civil War as well as the Sith Civil War. This is a logical assumption, since it would ensure the continuation of the "Darth" line of Sith for millennia to come until it came back into prominence with Darth Bane.