Its spoken of in prophecies.
"Its teeth are strong/ the tongue sups stars/Stars are nectar/Bars are long/Barred teeth crack worlds." The Prophecy of Questor Radcliffe the Solemn. (Mnemonic is alphabetical, PQRS.)
"From beyond it comes, and when it rips open the doors, no man, though armed with the divas sword of fire shall stand against it. The stars will wheel from their orbits, and all flesh shall fail save for those who have their trust in the Merciful One. They shall be saved beyond the beyond where even the Great Wyrm cannot go." The Rydlles of Kot, priest of the Merciful One in the Reign of Haus the Overlord. (Mnemonic is Kot=Cot for Cottage and Haus=House).
The Alchemist upon seeing it stood and stared for a week at the behest of the Star Emperor, and then he turned to the Emperor and spoke but six words. "You need to begin evacuating now."
There are other possibly more destructive monsters. The Deathbird flies every ten thousand years in its local universe, and visits dozens of other universes to lay an egg. Each egg is a practical dimension bomb from which another deathbird may be born. Some speculate that that deathbird was a kind of Cosmic Clean-up Squad for used up universes that went awry with the entrance of evil into the Multiverse. Instead of dropping off an egg,a nd the egg waiting until the End of Time to blow, it blows when it jolly well pleases. Bye-bye dimension.
And the Altairan Beast of Coils of the Galactic Supercluster of Hercules (mnemonic ABCGsH) has a spiral galaxy for its left eye and an antimatter spiral galaxy for its right eye. Its thoughts are passed through nerve connections formed of tachyon bursts and intergalactic jump gates, but it still takes a year for it to think "I" and another year to think "Want". Its body is formed of nearly five hundred galaxies, and its quite a bit larger than the Massosaur.
But less deadly, and smaller, and still people from all over the Multiverse fear the Massosaur far more.
Its length varies from One Astronomical Unit to One-Two Lightyears (Much of the data on this creature is uncertain, and was gained at the cost of heroic sacrifice). It has the head of a dragon with ten thousand mile long antennae sprouting by the dozen from its armored face plate. This head is sufficiently armored that direct contact with a galaxy of antimatter would seem likely to do no serious damage. No one has tested this theory as it seems to have the ability to change its atomic spin to become positive or negative matter at will.
Its tongue is quite capable of sucking up a sun from the outer edge of a solar system. This is a tube which can extend at a minimum of a quarter lightyear.
Direct nuclear explosions on its armored head do not seem to do more than slightly polish its rather dirty and encrusted hide. Some scientists claimed that it 'enjoyed the thousand missile strike we hit it with, as if it were a soothing scrub'.
An attack on a dimension happens thus. The antennae penetrate into a dimension and various tests are conducted to make sure the targetted dimension is suitable for a Massosaur visit. One key factor is if the Bias level is high enough. The creature could conceivably be stranded if it landed in a dimension with insufficient bias for its powers to be consistently effective.
Upon a positive report, the Massosaur exerts its 'teeth' which cause massive gravitational ripples, and rip a gate into a new dimension. This gate is typically at a minimum a planetary diameter. It then sticks its head in, and 'corks' up the hole with its head, and its neck ruff.
A period of quiescence then, usually a couple weeks as the Massosaur recovers and looks about. It then sends its tongue to get a nice reviving drink from the local star.
Perked up, it opens its mouth (about a month to two months after arrival, and two to three months to even a year since the antennae first checked in) and it begins to exert a very strong gravitational force along with other unknown forces. The dimension begins to shrink in size, and the gravitational forces move the stars, galaxies and such toward the maw of the massosaur. Eventually it all goes down the maw to be digested.
Evidently it is far larger inside the Massosaur than outside, or some other effect is happening. The 'material' makes its way down, gets digested in each of the first four stomachs, and then squished down toward the end in the last five passages, and then excreted in a small bubble universe that the Massosaur creates.
This bubble universe is typically the size of Jupiter, and it contains 99.99% pure entropy. It has -15@10 Bias across the board.
The Massosaur is shaped as a series of connected oblong balls about the size of Earth, with a head twice the size of Earth, and a ruff about the size of a small gas giant. It has a spinchter half-sphere at the end. There are ten parts to the Massosaur, that is the head, eight body spheres, and one half-sphere.
Each part is separated by a sub-universal boundary.
Inside the gut of the creature, the sub-universal boundary largely acts as a filter to keep certain 'proteins' from the digestion where they are needed to be, and its fairly passable. On the surface of the creature, the sub-universal boundaries tend to require finding very difficult passageways amongst its ravine like armor skin.
And since it has certain flying bees of enormous size that stitch these holes back up, you might need to find a new one every few months. The bees are symbiotic.
Another symbiotic species is a rather incurious race of dwarfs who mine certain very dense rocks from its skin. This has the effect of 'popping acne zits' and relieves the creature of a minor irritant while giving the dwarves their primary metal.
These dwarves can be found everywhere but on the head and the tail. The head is the 'warhead' and nothing is allowed up there. The tail is so vile that nothing can live down there.
The RuffShield is the Ruff and the effect it has on the subuniversal boundary outside the Massosaur's skin. It makes it very hard indeed to break through. This Ruff also generates side lobes that make it rather hard to 'catch it in the flank' as it were by jumping to a side dimension and attacking it in another dimension further down its body.
Each of the subuniverses (except for the tail) has the capability to merge into and hook into another universe. So the Massosaur is typically in nine universes at one time. This enables him to resist such tricks as the one where the Emperor of the Sun, a noble samurai, chose to do his duty and destroy his universe with a black hole device to also suck in the Massosaur. The Massasaur barely escaped, and would not have without its small claws along its sides dug into the fabric of other realities.
This does mean that of the nine dimensions that see the Massosaur, only one gets eaten. Typically having the Massosaur 'pass through' does cause significant collateral damage with instabilities in the space-time continuum, breaking of ancient spells, collapse of warp capability over a galaxy, and such like.
Also, the Ruff Shield has a side effect. People or things from nearby dimensions may suddenly find themselves on the exterior of the Massosaur.
There are varied civilizations on the Massosaur. Its a hodgepodge. A starship crew here, an ocean-going raft society there, a hockey team here... Some have been there for decades,and even a few for centuries. Others got here last week.
Each of the 'spheres' has the same progression. The sphere behind the head has 15@10 across the board. The next 14@10. And so on down to the one just short of the tail which has 7@10 across the board.
One advantage of this type of creature is that it gives super-versers like the Alchemist and Michael Di Vars something to seriously challenge them. The Multiverse is vastly larger than even an uber-powerful verser, and you can just invent a new larger version of dragon for the hero to take down.