The Defeats of the Chieftains

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?

Erak'tab my man is probably chilling in Lake Keni-po and then once the city was raised from the water took a hop in and is actually controlling the elemental guys~!!!

Seems a little grand for him. Yeah, he was Aa'une's top guy according to the book, but he didn't really display it well, being among the first of the Chieftains to fall and not getting a promotion like Ihun'kalin, Fal'makin, Milla'iin, or Rath'tab

That would make for some interesting plot development though But you are right, it doesn't seem very likely.

If a M'arrillian would make a return, Mock'adyn is the one with the most set up. He's the only one with a card specifically detailing that he's attempting to wrestle his way into power. The only issue is that he's not a Chieftain, so perhaps Fal'makin could do it.

Back on topic though, me personally, I am of the opinion that Tartarek defeated Erak'tabb based on Carnivore's Keening. However, Carnivore's Keening could also be the moment when Tartarek fell, as it's still early in the invasion and it would synch up with PSI Overloader's timeline. So I don't know, I just like the idea of one of Erak'tabb's brainwashed minions(Rellim?) being able to break his control and defeat him

I have to agree that the fact that a second version of Mock'adyn means that the M'arrillians weren't entirely defeated.

You see, their leader controlled and maintained the tribe. Once he was defeated, the others that made it out alive... should still be alive as free-lancers and should be on their own accepting their lost.

I don't think the survivors would try to lonewolf it, the ruling structure is clearly an oligarchy and council of chieftains. Given the M'arrillian propensity towards always evil(I have not seen a single M'arrillian card that doesn't have some level of sinister to it), it's more likely they'd try to take Aa'une's place, fighting over the scraps of what's left.

They could have had a system in place for the possibility of Au'une's defeat. I feel like the M'arrillian's are an organized enough tribe to have some for-planning. But I doubt they expected to have lost as much control as they did. Which is why they have not made an immediate counter-attack.

They may have planned for the death of maybe one or two Chieftains, given Ihun'kalin's lack of care over Erak'tabb and Neth'uar's failure. But all together counted, that's 5 Chieftains either dead or defeated. To make matters worse, Erak'tabb was explicitly said to be high ranking, Milla'iin was Aa'une's aid, and Aa'une was the outright acting leader.

It would be dark times when the Inquisitor(Fal'makin) and an Elementalist(Mock'adyn) are the best candidates for leadership.

EDIT: Upon looking at the timeline, I think the M'arrillians are better off than I thought. Given that they are still around after Aa'une's demise, I see some leadership. We know Milla'iin, Fal'makin, Ihun'kalin, and Rath'tab got promoted(though Rath'tab got exploded in the very next set). More importantly, Ihun'kalin says that Erak'tabb's and Nethu'ar's fall just mean higher stakes, in a card with a rampaging Kha'rall. Fal'makin seems to have a Kha'rall under his command as well in Ast'imal. So the Chieftains are either blinded by arrogance, or think they still have a chance of winning.

On another side though, we know Mock'adyn is attempting to do a hostile takeover and Thar'lyn, one of Fal'makin's boys, is spying on Fluidmorphers.

I guess Mock'adyn is rallying the Fluidmorphers(according to Siril'ean's entry in the book, they were viewed as useless before invading Perim. That could give motivation) and Fal'makin doesn't like that. We see Ast'imal roaring at a Fluidmorpher, so I think Fal'makin is the biggest player besides Mock'adyn for leadership. Ihun'kalin and Milla'iin would be too busy trying to defend the remaining footholds.

Random note, I guess Bodal's lava trap failed as the Mugic card that had that happening was released before Milla'iin, Foothold Commander. Guess Bodal messed up on that one

I always thought within the show's universe (I know it's different from the cards in a lot of ways) that Rath'tab and his squadron of Coral fighters were frozen in the tidal wave because the last shot you see of them is the tidal wave overtaking them as they cheer.

Given Rath'tab is basically liquid in armor, it's possible he could squirm his way free. Aa'une wasn't dead at that point and so the Chieftains were practically invincible, according to Aval'par