Let’s not mince words: Mogaru is Pathfinder’s Godzilla.
You read that correctly. GOD. ZILLA.
Naturally, there are cosmetic differences—Mogaru, known as the Final King, has two tails, for instance. But the essentials are the same. If you have ever thought, “Man, my magus is so powerful she could take on Godzilla,” welp, now’s your time to put up or shut up.
Pro tip: Bring the bard along. Make that pretty boy be useful for once.
I’ve never actually seen an entire Godzilla movie—my media teacher who first exposed me to monster movies stuck with the Universal monsters, and even my viewing of Godzilla’s most recent incarnation got interrupted. (Godzilla is no match for hospital visiting hours policies.) But I know enough to know there are all sorts of adventure styles and themes you can explore with Mogaru, from straight-up “Kill the monster” adventurers…to luring Mogaru into fighting another kaiju or similar big baddie…to exploring themes of environmental or magical devastation…to simply fighting evil aliens, outer dragons, and other threats from beyond the stars. Pick your favorite Godzilla movie to use as inspiration and start designing.
One last note: Kaiju, like dragons, are assigned an element—Mogaru’s is water. Aside from a handful of classes (like the elementalist mage) and a supplement or two (like Blood of the Elements), elemental ties haven’t been explored much in Paizo. But I can imagine campaigns where the elements do matter—where Mogaru might have more in common with, say, marids and black and bronze dragons than he does with other kaiju. I can imagine campaigns where elementally aligned PCs may have to try to rouse and direct Mogaru—or destroy him—based on the dictates of their elemental bloodline, house, faith, or school of magic.
Adventurers fought the void dragon O’tar’nesshth—and lost. Now the dragon known as the Last Aurora lords over the capital like an alien god. As the adventurers try to collect resources and allies for a second attempt, word reaches them that a beast from legend has awoken—and it is headed straight for O’tar’nesshth.
Marids are capricious, chaotic folk, mirroring their favored ever-changing element of water. Most sages suppose this is the reason that their race boasts so many shahzadas, yet cannot unite under a shah. But there is another reason—a curse—that binds the marid race. Whenever a single marid ruler grows too powerful, the Final Shah awakens from his watery slumber to remind the genies that the Plane of Water has only one king. Which brings us to the present day’s crisis: A pending marriage could bring three planes and fully half the marid race into an alliance. Adventurers are hired to disrupt the nuptials before Mogaru wakes.
Mogaru is renowned for his hatred of air kaiju—his many battles with Agyra alone are legendary. But does his hatred extend toward other great Powers of Air? With the vrock armies of the Abyss pouring into the skies above the Serpent Gulf, there’s no time like the present to rouse the great dinosaur and find out. Especially when the vrocks’ general shows up: Pazuzu himself.
—Pathfinder Bestiary 4 170–171
PaizoCon happened this weekend, and like you I was watching the announcements as they poured forth. To answer your question: Yes, I am whimpering with terror at the announcement that Bestiary 5 is coming out, with 300+ new monsters in tow. Then again…those preview images look pretty dang cool…