Most of the Great Generals of the Dark Ones were former warlords of Dark One tribes. Some of them were once hated enemies, and some of them had fought against each other for other reasons. After NiTian ErXing reunited the Dark Ones and brought an end to the Dark One's Civil War, many of them came to the realization that they had always hated the days when they were forced to kill each other. A pact was made, the thirteen Great Generals promised to let go of their past grievances, and from that point, started calling each other - Brothers. 

"Brothers do not fight one another, and when the time comes, they will gladly die for each other."  This was the promise the Great Generals had made.