Ok I need help fleshing out a campaign idea that I have been playing with for a while.

• The premise and principle is a Tarrasque is unearthed by mind flayers.
• The mind flayers knowing that Tarrasque weakness is that there is only one clone it and mutate it to be smaller and faster.
• Oh, and the mind flayers bolted on their over mind on to the original Tarrasque.
• The adventure so far is this
  • The advetures start in a major city. They do some down time activates and in the during the night a massive earth quack happens. They go full disaster relief and adventure in rescuing trap civilians. Then a few adventure happen and then the high elves suddenly have Drow refugees lead by a naïve priestess showing up from the underdark. They bring word that the dark elf capital has fallen and their leaders are fighting a running battle fighting Tarrasque mutants and losing. And this is where I am drawing a blank.
  • so any help would be appreciated, please and thanks  

Back up a bit: Could an Elder Brain even control a Tarrasque? Would an Elder Brain even want one? Tarrasques destroy. Illithids conquer. It seems like madness.

Then again, I see no reason why an Elder Brain couldn't succumb to madness just like any other mind. Given all the experiences and personalities it absorbed, it could have concluded that life is the aberration and shouldn't exist. It could simply be sick with a disease of some kind.

Imagine curing an Elder Brain of its madness and it (with its colony) has to assist you to undo the damage it's done to its potential rule... but what happens after the alliance is no longer needed?.. What would be the reason the Elder Brain, after being cured, wouldn't simply enthrall the player characters? I'm sure there can be a reasonable explanation, but an Elder Brain does not tolerate any examples of rivals for long. Its power must be absolute, which means it must eliminate the player characters once their usefulness is gone.

Quote from EricHVela >>

Back up a bit: Could an Elder Brain even control a Tarrasque? Would an Elder Brain even want one? Tarrasques destroy. Illithids conquer. It seems like madness.

Then again, I see no reason why an Elder Brain couldn't succumb to madness just like any other mind. Given all the experiences and personalities it absorbed, it could have concluded that life is the aberration and shouldn't exist. It could simply be sick with a disease of some kind.

Imagine curing an Elder Brain of its madness and it (with its colony) has to assist you to undo the damage it's done to its potential rule... but what happens after the alliance is no longer needed?.. What would be the reason the Elder Brain, after being cured, wouldn't simply enthrall the player characters? I'm sure there can be a reasonable explanation, but an Elder Brain does not tolerate any examples of rivals for long. Its power must be absolute, which means it must eliminate the player characters once their usefulness is gone.

 Well the question that I was trying to figure out what would the Elder Brain respond to finding a Tarrasque? It would be a deceive resource if exploited. 

I think the elder brains plan would involve only the original Tarrasque. Just let it go and absolutely destroy the kingdom's defenses. Then, the Tarrasque gets full and goes back to sleeping underground somewhere. After that, the elder brain can invade.

The Elder brain likely wouldn't try to control the ORIGINAL tarrasque, as it is too powerful and unpredictable to risk having the elder brain too close. However, more obedient mini-tarrasques, with some of the power and a link to the elder brain could assist in the invasion.

So, in conclusion, the elder brain wouldn't try to dominate the original tarrasque. But everything else about this plot is great.

Well, this isn't the first weird Elder Brain idea I've seen (I've considered having some sort of beholder-elder brain-aboleth abomination). I agree that controlling the Tarrasque is idiotic. But nobody said the Elder Brain has to be fully sane.

If it did possess the Tarrasque, there's already a stat block for it.

https://homebrewery.naturalcrit.com/share/SyZcaPl8-Z

eyrieking162 invented this and put it on reddit.

https://www.reddit.com/r/UnearthedArcana/comments/6dj6ez/the_elder_brain_tarrasque/ 

This is terrifying.

from a theorycrafting POV control is possible, simply thorw enough Intellect Devourers at the Tarrasque and it'll eventually be reduced to zero Intelligence and become stunned until it can regain a point of Intelligence, in the mean time the Elder brain introduces a metric ton of Illithid tadpoles into the Tarrasque and the mindflayer colony spend much of the time psychicly transforming the Tarrasques brain until such time as the tadpoles transform the Tarrasque into a Half Illithid under the control of the Elder Brain. The Elder Brain can then pplane shift the Tarrasque to whereever they want to wreak havoc, destroy defences and generally lay waste to everything before the mindflayers invade on mass to clean up and the Half Illithid/Tarrasque is Planeshifted back to its lair until it is needed again.

Quote from Rob76 >>

from a theorycrafting POV control is possible, simply thorw enough Intellect Devourers at the Tarrasque and it'll eventually be reduced to zero Intelligence and become stunned until it can regain a point of Intelligence, in the mean time the Elder brain introduces a metric ton of Illithid tadpoles into the Tarrasque and the mindflayer colony spend much of the time psychicly transforming the Tarrasques brain until such time as the tadpoles transform the Tarrasque into a Half Illithid under the control of the Elder Brain. The Elder Brain can then pplane shift the Tarrasque to whereever they want to wreak havoc, destroy defences and generally lay waste to everything before the mindflayers invade on mass to clean up and the Half Illithid/Tarrasque is Planeshifted back to its lair until it is needed again.

 And then they build their colony on the back of the Tarrasque. Dino-mounted castle of doom and mind flaying.

I realize this is necro-ing, but I just had this thought myself recently and wanted to throw in my 2 cents. 
My idea is that an elder brain going by Katulu was able to take control of a tarrasque. Using the fake name Cuthulu, it has started cults dedicated to itself as an ancient and lost god. It hopes to used the divine spark that all mortal souls have to harvest devotion and worship from it's cult followers and make it a god. With this divine power, it will be able to challange and destroy the lich-queen Vlaakith and bring about the Illithid empire with itself at the helm. 

Intellect devourers can control a human by entering their skull, and they are just brains subjected to a ritual. ergo, you could do the same ritual on an elder brain, creating an elder devourer, and using it to control a tarrasque