(I made this clan up. Please don't hate me!!! I tried!) 

This clan isn't like any other clan. They still live in the trees, but they do not worship the same all-powerful mother, Eywa. Instead, they've grown dark. They've twisted nature into horrifying magic. They realize that the all-mother had been keeping some magic hidden. Horrible dangerous ways that can destroy the world. This clan can summon the Black nì'awve, the Black Fire. They use it as a weapon to destroy and conquer. 

Thousands of years ago, they decided that they were better than all the Navi clans. So, they launched the Black nì'awve. It consumed the land. It consumed everything in its path. No one could stand against them with this weapon. Then, the other clans turned to Eywa for protection. She gave them a golden bottle. A bottle that can suck their weapon and destroy it in the Bottle. 

They used it. The Black hì'i 'eveng were powerless against the weapon of Eywa. So, they fled and most were hunted. There's said to be no more of this forbidden clan.

 However, some say Agallia the High priestess and the creator of the fire had escaped. Her powers are dark, and she was so powerful. She could summon the dead.