Need help creating a boss!

I am aiming to wrap up a campaign sometime soon and I need help creating a boss the idea is:

There is an Eldrich god trapped beneath the sea floor, as the party arrives it escapes then the Kraken arrives and forces the god into the cavern it was once trapped giving it's life (the Krakens). Then the party will go into fight it ending the campaign they will be level 5-6 there is a barbarian, artificer, 2 rouges and 2 others I can't remember (we are on break for summer ending in the next 2 weeks)

For level 6, a Kraken itself would be devastating. If you're saying this Eldritch god consumed it's life essence, then I'd say the god needs to have the ability drain lots of HP to empower itself. To make it interesting, you could make it an action that requires 2 failed saves to work. 1st fail: target is Slowed, 2nd fail: target loses massive amount of HP.  The god could then get Advantage on all attacks for a minute, and gain some abilities.  

After draining the kraken, the god could have the ability to make 3 tentacle attacks, or summon a lightning storm.

Sounds way overpowered for a party of lvl sixes, so I'd recommend adding a way to weaken him, follow video game logic, if theres a really powerful boss, drop some stalactites on it, push it onto a thermal exhaust vent that lowers it's AC and gives it the stunned condition. You don't have to do what I'm though it would be helpful if you still want a powerful boss that your players can beat. And if you make the boss and your players start to lose, you can even throw them in there later if they pass a perception check after they start losing. Especially if you want a satisfying ending, make sure you don't have the environmental conditions kill it, let your players finish him off and make sure they can plan around any environmental conditions so its not just "randomly a stalactite fell on it, dealing 3d12 piercing damage". You do you though, and just make sure that you have fun. Good luck

Well you need to come up for a reason why the god is pathetically weaker once in the cave compared to what it was that the Kraken was fighting. Because a Kraken would TPK a party of level 5-6 with a single action. How about something like this:

The cave is a magical prison for the god, it is covered in glowing runes which magically negate most of the god's powers. While the god is magically strong, it is physically weak being a creature made from the fears and terrors of humanity itself rather than a being of flesh and blood. Hence the Kraken is able to physically push the god back into the cave, but the god is able to curse the Kraken that will kill it over a short amount of time - this gives the Kraken time to explain what is going on and what the party need to do to permanently seal the god back into the cave.

Now the last conflict is not actually fighting the god itself as it has been restrained by the magic of the cave, but instead the god uses its mental powers to control members of the party and/or nearby sea creatures to attack the party while the party is trying to complete the final steps to complete the seal that will seal the god away forever and block its mental powers. Completing the seal might require collecting a set of perfect pearls from giant oysters and attaching them in a specific pattern around the cave, or it might require going through the cave & cleaning & repositioning a series of diamonds to focus a beam of magical light onto the god, or it might require finding a specific object that came loose and fell to the ocean floor and putting it back into its holder. 

So if I understand correctly, the situation is: The god is trying to escape its cavern beneath the sea floor. A kraken showed up and shoved the god back into the cavern, dying in the process. Now the party is there, and the kraken is dead, and they have to keep the god from escaping again. Right? 

My instinct here would be to set up some sort of mechanic or ritual that the party needs to do in order to seal the god into the cavern. So like, the seals failed and the god escaped. The kraken showed up and shoved the god back inside, but to keep it in there the seals must be renewed. (Or something.) Then the boss can be whoever/whatever is trying to stop the party from doing that. 

This way, you don't need to have your party fight a nerfed god. Mechanically that would be easy enough. Just pull like a maybe CR 10 stat block so it's a hard fight, say that in this cavern the god is weak and can be confronted, and let it rip. (Use Kobold Fight Club, I'm just ballparking on the challenge rating since you've got 6 players.) That certainly might be possible to pull off narratively, but my instinct is that it would be hard to make this feel satisfying for the players. ("So... we killed god with a +1 dagger? Cool, I guess.") 

Instead, I would say look at your story. Why is this god escaping? Does anyone/anything want to help it? Who have they been fighting so far to get to this point? I'd use them as your boss. Have an epic fight with whoever your party has been fighting along the way as they try to stop your party from sealing the god back in the cavern. That way, you can have a great boss, and the god can remain a looming, enormous threat that gives this a sense of real stakes.

I'd say that at this level, you don't want to see your players fighting an eldritch god. They're just too weak for anything so very big. Remember at Tier 2 they're local/regional heroes. They are saving regions or small kingdoms, not the entire world. By level 5 they've only just gained a mastery of the basics of their classes and powers. They've got a long way to go before they're world saving heroes. I'd say that the adventure has been somewhat poorly paced. There's a reason why Strahd's goal isn't to turn the entire world to vampires. He's a low level dude looking for a wife, successor, or consort. His goals are small, his reach perhaps even smaller in the grand scheme of things. By the end of Curse of Strahd, the party have 'saved' barovia maybe, not the entire world. By this point the characters are barely level 10. So my answer reflects that, your characters aren't world saving heroes by level 5-6, they're baby adventurers who've saved a town, a village, or put a stop to a minor villain before they could hurt more people.

My suggestion would be to have them fight a Sahuagin Baron, 2 Sahuagin Priestess, and 2 Hunter Shark. Have the group be attempting to summon the Kraken. The stakes of the battle are quite clear. If the Baron is stopped, the Kraken isn't summoned and the party are victorious. The big threat of the campaign to date is over. If they aren't victorious, the Kraken is summoned, the eldritch god arrives and the party are simply too weak to defeat it. They get shipwrecked or knocked out, waking up later with the clear goal of becoming powerful enough to actually put a stop to the Eldritch god before it can destroy the continent, and then the world. There is of course the third option of the entire party being killed by the eldritch god if they fail, but that ought only be used if the group is tired and wants the campaign over and done with.

I agree on the point that they're level six, so maybe it doesn't make that much sense to fight a full blown god, but maybe that's just me...

Perhaps something like the end of the Rise of Tiamat, where their is a ritual being conducted by a cult in the corpse of a slain kraken who died centuries ago trapping the god in the cave, and the party needs to fight the cultists to stop them from conducting the ritual, and can also weaken the ritual as they do so. If they fail to stop the ritual, a vestige of the god, weakened by their tampering, comes through to fight them. Makes more sense that way, and gives them multiple ways to go about stopping the god's plans.