So I'm making this monster called "The Beast", it's supposed to be stronger then most other monster. It comes from far realm, which gives it an excuse for being strong and powerful. It can use psionics and telepathy, it has powerful regeneration and healing powers, and has both tentacles and hands so I think that is good. I tried basing the stats of the Beast off of both the Tarrasques and a kraken but I think I jumbled it up a bit. What i'm looking for is an all powerful final boss type creature, but I feel like I'm trying to hard by constantly adding things. So what should I keep or take away? Heres a list

-Healing powers. It can heal itself in combat

-Immune to all weapons, has resistant to magical attacks, extremely vulnerable to a magical sword made to defeat it.

-Telepathic powers it can use on enemies like the mindflyers just without recharge. It can sense things like being upset.

-Shapeshifter, one that can mainly go from garagantum to large, and morph certain parts of it's body. Such as growing tentacles, claws. Changing it's face from a scary monster face to a humanoid.

-Attacks count as magical ones. 

-Can summon sudden weather changes, meators, lighting, rain, etc.

-Gets muilitattck attacks.

Why?

A Tarrasque  is a deadly level combat encounter for 6 level 20 characters already, and still a Medium level combat encounter for 12 level 20 characters.  How big a group of 20th level characters do you have?

The Tarrasque is already an example of a monster that really can't be killed - and it's kind of an "in joke" for that reason. It's really just a design exercise, not a practical opponent. Your creature would just be more of the same.

If you're just playing with the system to see what's possible - that's one thing - but I don't think such a creature would have any practical game value,

So just base the stats off of the tarrasque and other monsters

All depends on what you're trying to do.

If you're trying to do this as a design exercise to see how deadly a monster you can build - then I'd recommend becoming very familiar with the custom monsters creation rules and "system" ( such as it is ) in the DMG. I'd also recommend checking out The AngryDM's Series on custom monster building ( brilliant content; infuriating writing style ).

If you're trying to make an end-of-campaign monster for your Players, my advice would be "don't" - such a monster has no practical application unless you have a party of 7-8 level 20 characters, or you really really want to TPK them at the end of the campaign ( which is - in my opinion - just a dick move ).

It seems rather absurd to build a creature that they have no chance of killing even if they have the "mcguffin of slaying".

Let's start with a section in the DMG that explains how to create a creature from scratch starting on page 273. Now we're going to look at the table found on page 274 for this hypothetical creature.

First, CR 30 is the highest number that we're going to be able to work with on that table. These are your base numbers:

Proficiency bonus is a +9
AC is 19
HP is 850 (max)
Attack bonus is +14
Damage per round is 320 (max)
Save DC is 23

Now we haven't even looked at what your Ability scores are yet which are going to affect these numbers as well. Then we have Size category which will affect your hit die, and then you have your traits, Legendary Actions and Lair actions, all of which affect those base numbers. 

So, looking at those numbers you're getting a baseline:
STR (20) since the +9 Proficiency bonus and the +5 from STR give you your +14 to hit. If you decide to give it an absurd strength you're to hit mod will increase along with it. 
DEX (?) if you say it has natural armor you can make DEX whatever you want and keep it at a 19 AC or increase it by whatever modifier you choose. If you go with a DEX (20) then your AC becomes 24.
CON (?) A gargantuan creature has a base HP die of a D20. If you want 850 HP you're looking at just shy of 81d20 but that is with a CON (10) since you're not giving any modifiers to HP. If you go with 81d20 and a CON (20) you're bumping the HP up to 1255 (81 d20 + 405).

These are just the basic numbers modified by Ability scores. Now look what happens when you give it Immunity to damage:
HP 1255 * 1.25 = 2510 effective HP

Then we add Healing, assuming it's similar to Heal we add that to it's effective HP, and we assume it has the same basic spell slots (3) that's another 210 hp it can get back
HP = 2720

And, if you add regeneration, we add another 50% of it's base HP to the pool:
effective HP = 3875

We haven't even looked at the damage it could cause and you can see that it's HP pool is HUGE. At this point in time the fight is just absurd. A Wizard casting Meteor Swarm at this boss, assuming it fails it's save, would only deal an average of 140 damage and a maximum 240 in total. Assuming a party of 5 dealing roughly 700 points a turn it would take about 6 rounds to kill this boss. That is before we even take into account the damage the boss is doing to the party. 

A Circle of the Moon Druid at 20 will have approximately 203 hp, plus able to turn into an Elemental an unlimited number of times. You would kill it in one shot after adding your modifiers to the damage. A Barbarian has around 368 hp while raging and Bear totem. Dead in two rounds after you add modifiers to the base damage, one round if say one of those attacks is Psychic. Those are two of the meatiest classes and they wouldn't last more than two rounds while it would take 6 rounds to kill the Beast. 

 I wasn't too far off :)

I said 7-8 level 20 characters would probably kill it, but it would most likely take 10-12,  and even then it'd be a bloodbath with only a few of the characters still standing at the end.

I was basing my guesstimates off of Kobold Fight Club calculator.

Quote from Vedexent >>

 

The Tarrasque is already an example of a monster that really can't be killed - and it's kind of an "in joke" for that reason. It's really just a design exercise, not a practical opponent.

And this is why I've resented the inclusion of the Tarrasque in every incarnation after the first. Why waste your precious space in my monster manual (particular in print) with a joke of a monster that almost no one is going to use anyway other than a drunken one-off some night? Blech. Give me another...well, anything. Half-orc-half-celestials, or were-werewolves (they sometimes turn into werewolves--not the hybrid, they just sometimes turn into people who might turn into hybrids), or the crazy tree-stump-rabbit-monster. Something I might actually use.

End rant. 

Quote from Fake2Name >>

So just base the stats off of the tarrasque and other monsters

 Yep. Thats what I would do. Its easy to come up with all sorts of concepts. I do this all the time when  making something unique to my game.

There a difference between making a custom monster for a campaign - I do that all the time ( in fact my Players are pretty much surprised if they run across something recognizable from the MM ) - and making a super-Tarrasque, which has no possible use except for TPK'ing most level-20 parties.

I could see cooking up a monster who's sole reason for existence was to provide the characters with something to run away from and have no prayer of beating. If the characters just saved the world from a demon invasion, having something literally 1/2 mile tall haul itself out of the ocean and start wrecking everyone's poop could be a nice moment for humility :) "Hey guys...not everything is killable!"

Like having their kingdom get hammered by a massive hurricane. The unkillable monster as plot device. But at that point...you don't even need stats.

Quote from Brotherbock >>

But at that point...you don't even need stats.

Yep - this.

You could do all the purely narratively - describe it in epic horrible terms, have it devastate the landscape, have it dish out horrific amounts of damage, have it absorb massive amounts of damage. But since it's literally not defeatable, you don't need to track its HP. It's attacks can dish out "whatever character it hits 1/2 max hit points", etc. You don't need to design a creature - you can just improvise awesome effects and horrible results.

The danger of a literally undefeatable monster, is that the Players might not get the memo - they may still try!  Players are there to be heroes - heroes don't back down - and suddenly you have a TPK.

How about making the tarrasque just the mount for something even worse.

And giving the tarrasque the ability to fly.

And the something even worse does psychic damage. You don’t want the Bear Totem Barbarian to have it easy.

A 6 member party of 20th level Tarrasques. Including a Bear Totem Barbarian Tarrasque.

Adding class levels to the tarrasque. I love it!

20th level Wizard tarrasque with Simulacrum.

Tarrasque Life Cleric/Lore Bard. Because sometimes you need to go against type.

Have them fight this: https://www.gmbinder.com/share/-L_yA4hT3lr4aJw-UgOp