From what I can see, here are the confirmed defeats
Neth'uar was torn apart by Odu-Bathax(Odu-Bathax - Reservoir Reclaimer)
Rath'tab was exploded by Arctus's Allmageddon attack(Arctus)
Milla'iin was turned into soup by Bodal's lava trap(Skysong Arpeggio)
Erak'tabb was defeated at some point and is mentioned by Ihun'kalin among the Chieftains who failed along with Neth'uar(Klik'ssi)
Aa'une was defeated at Lake Blakeer by Maxxor, Chaor, and Iparu, leaving a power vacuum(Mock'adyn - Fluid Master, Aa'une Avatar Alt. Art, and Lake Blakeer)
What do you think happened to Erak'tabb the other Chieftains?
Neth'uar was torn apart by Odu-Bathax(Odu-Bathax - Reservoir Reclaimer)
Rath'tab was exploded by Arctus's Allmageddon attack(Arctus)
Milla'iin was turned into soup by Bodal's lava trap(Skysong Arpeggio)
Erak'tabb was defeated at some point and is mentioned by Ihun'kalin among the Chieftains who failed along with Neth'uar(Klik'ssi)
Aa'une was defeated at Lake Blakeer by Maxxor, Chaor, and Iparu, leaving a power vacuum(Mock'adyn - Fluid Master, Aa'une Avatar Alt. Art, and Lake Blakeer)
What do you think happened to Erak'tabb the other Chieftains?