Okay, here's a quick rundown:
The hero is a two-part one: part-spirit, of a dinosaur King of ancient Pangaea. Think tyrannosaur, except larger, armored like the club-tailed Ankylosaurus, and with proportionately sized arms. Very intelligent, immortal(as far as age), fought many battles over the five million years of his reign.
In the story's prologue, he's hunting down a rogue spirit. Sometimes these spirits just lose their way or maybe something external turned it all dark and sinister. Anyway, it's a spirit of corruption, and deathly corruption at that. It possesses the corpse of a slain sauropod. Sauropods, for those who don't recognize the term, are the giant long-necked dinosaurs.
The two fight, and ultimately die. The Cretaceous ends, their spirits are trapped underground. 65 million years later, events conspire to awaken these spirits. Each "possesses" the body of a man. They do it in different ways, though: Gresh basically "eats" the soul of the man he possesses, whereas the King merges with Ash's spirit in what ultimately ends up being a consensual fusion.
The villain, Gresh, looks something like a zombie Undertaker: rotted flesh, clad in black, nasty smile. But, in the final battle with the hero, he is badly damaged and basically melts. Then, one by one, his undead legions cast themselves into the quivering mass of gelatinous dead flesh, and it begins to grow.
Ultimately, Gresh turns into a huge monster something like a sauropod, like the form he wore back in the Mesozoic. Long neck, horned head, but built even more powerfully than your typical sauropod, since he's stronger than he was. At one point the King rips off his head(which he defeated the monster before with), but then the entire front half of Gresh splits and ribs shift to form vertical jaws with horizontally-lined rib-fangs. It adopts a bulldog-like stance.
This thing is huge, too. Sixty, seventy tons, easily. The largest dinosaur was Argentinosaurus, weighing in at 100 tons(or more!), and this thing is nearly that size. Like a dragon, Gresh breathes fire, a black flame that burns like fire and melts like acid. Very powerful, a real monster. He's pushed into this form after gettin' into a slugfest with Ash, and then the two battle in the smoking ruins of this little mountain town.
Now one thing I always figured about the hero/dinosaur/phoenix is this: dinosaurs evolved into birds, right?. It's fact, it was proven long ago. Specifically, it was the theropod dinosaurs(T-rex, "raptors," Coelurosaurus, etc.) that evolved. What would the mystical King of dinosaurs evolve into? A Phoenix makes the most sense, doesn't it?
So it might make sense that the spirit of the King, trapped below ground after his physical body was destroyed, might evolve over time. When it joins with Ash, a human, the evolution is completed and made manifest.
And don't worry 'bout Harry Potter, folks. This Phoenix would be a giant bird of golden fire and thunder, similar in size to the King's dinosaur form(which stood ten meters high). And while he's intelligent, he has lost absolutely none of his primal ferocity, the savage theropod lust for battle. Facing his polar opposite, a creature of eternal death, the King knows but a single way to proceed: kill or be killed.
*****
And just because, here're some lyrics I always think of when I think of the transformation, from "Wait and Bleed" by Slipknot:
I wipe it off on a tile, the light is brighter this time
Everything is turning blasphemy
My eyes are red and gold, the hair is standing straight up
This is not the way I pictured me
I can't control my shakes, how the hell did I get here?
Something about this, so very wrong
I have to laugh out loud, I wish I didn't like this
Is it a dream or a memory?
Fitting aesthetically, as well, because Ash's eyes burn with a red-gold light when he's angry or powered-up.