Do'h - well I actually missed what CR you made it, I just focused on answering your question.... Sorry. But yeah, I think it's good.
In general, I think you should aim for your creations to be within one or two of the the MM's CR for those monsters that are replacements and not variants of WotC monsters. One point of the hardcore monsters is to beef up the performance within the given CR, right?
Caliburn101, thank you for the comments; however, you have me a bit confused with your comment. A have a couple comments / questions:
1) There are no official rules for levels beyond 20. There are guidelines for play beyond 20, but not levels. So, I don't really know what a level 30 wizard is. There are of course at least (2) 3pp products, but I am not familiar enough with them to comment.
2) My concern isn't 20+, but 20th level and below. I don't really want T to be able to be defeated by the average 4 person lvl 20 group.
3) Why use meteor swarm? Is about the worst use of a high level spell. T is immune to fire and has resistance to bludgeoning, plus its magic resistance.
4) I did a quick check and a party of 20th level PCs consisting of a cleric and 3 fighters can do enough damage to kill T in about 6-7 rounds. The trick is staying alive that long. They would need some extreme equipment (I had the fighters with belts of storm giant strength, +3 plate, +3 shield, +3 greatsword), but that is what I think it should take for lvl 20 PCs to take on big T.
1) Read the epic advancement rules in the DMG.
2) 20th or below? I have just shown how 4 30th lvl damage-specialised wizards cannot kill the T. with the best (by far) damaging spell in the game...
3) Other spell will be less effective the way you have this monster statted - check them all out. If you don't agree - name the spell(s).
4) As a GM I would make mincemeat out of a party of that composition with this boss, frankly.
Perhaps not intended, but your post seems oddly hostile. Anyway, here are my responses:
1) I have read the DMG and there are not rules for extra levels (perhaps you should re-read the DMG or my post again). There are rules for advancement beyond lvl 20, but it based on epic boons (with a variant to take a feat or ASI instead). In fact, the DMG states: "An epic boon is a special power available only to 20th level characters." They are specifically for 20th level characters, not 21 or higher. Also, you mentioned that your theoretical 30th level wizard had nine 9th level spell slots; however, the epic boons state: "Unless a boon says otherwise a character can't gain it more than once." And the boon of high magic does not specifically mentioned that it can be used more than once. So, by RAW there are no 30th level PCs, and a wizard cannot get nine (9) 9th level spell slots.
2) Yes 20th level, the official highest level of the game. Again, meteor swarm is the highest damage spell, but it is a poor choice against big T. (he is immune to fire and has resistance to bludgeoning). MS does a max of 35 avg. damage while a 9th lvl disintegrate does 106 avg. damage.
1) OK then - the wizards die even faster - check.
2) Disintegrate is a RAY - and you don't want to go using rays on Big T.....
3) So what? They run out of effective spell slots a VERY long time before Big T. has so much as a scratch on the paintwork!!
Also - you have the three Fighters at +20 to hit...
+6 for Prof. +5 for Str, +3 for artefact weapon... where does the other +6 come from?
You have in this one case created a white-room challenge that needs precisely the party you describe to have a vague chance against it - every other choice is worse, and offensive spellcasters are absolutely useless in this fight - completely, utterly ineffectual.
There is no point making a challenge no campaign will ever use I would say. Big T. needs seriously toning down if it is to be anything but a curio in the otherwise really very good line-up of monsters you have created.
I would use most of the monsters here, and they would be interesting and challenging additions to my game. Big T. would never get a showing, unless I wanted simply to end the gameworld in a deus ex machina way.
The reason it's written that way is because the game only officially supports 20 levels.1) I have read the DMG and there are not rules for extra levels (perhaps you should re-read the DMG or my post again). There are rules for advancement beyond lvl 20, but it based on epic boons (with a variant to take a feat or ASI instead). In fact, the DMG states: "An epic boon is a special power available only to 20th level characters." They are specifically for 20th level characters, not 21 or higher.
The reason it's written that way is because the game only officially supports 20 levels.
Using epic boons to offer epic progress (beyond the rules in the PHB) seems like a perfectly normal thing to do.
All three fighters have girdles of storm giant strength...
Good lord - what campaign is this supposed to be IN?
The point stands. This is a whiteroom monster and no actual campaign would warrants it's use UNLESS as a world ending monster, or in the extreme corner case that from low level the party knew it was coming, had somehow divined or researched all of it's abilities and dedicated their careers to being the ones to kill it.
Every other entry here is usable in any campaign as a challenge to whatever party of heroes exist. Big T. does not.
If it is a world-ending monster a GM needs - it's good to go. If not, then it is a curio in the line-up never to be used unless there is some other way to permanently trap it that exists in the game as a unique artefact or something similar - a kind of 'you cannot kill this so you have to get very creative' sort of way.
In which case, I'd put that in the description for the GM to get a sense of how it is best used. 5th Edition is very magic-centric and this is too anti-magic to work for a normal campaign.
Never underestimate the power of high level groups...players think of the most insane ways to beat monsters.
Case in point. I see nothing that would stop a party from casting plane shift on the Tarrasque to the Elemental Plane of Water (its even relatively easy as far as things that hit the creature go).