Jörmungandr
Sea serpent from Norse mythology, son of Loki
Why?
By definition, there could be only one Apex predator in one given environment.
Jörmungandr is a marine creature. Problem solved.
Classic fantasy based, preferably Western culture originating.
That's Norse mythology. Unlike Leviatan that is Hebraic.
They can't be a standard biped race, humans and elves are so last millennium
It have no foot.
And just like @itcouldevenbeaboat 's Angel, he is literally from the last millenium as he'll come to poison the sky in the Ragnarök time.
Not a dragon or a dragon clone
A sea Serpent. You judge
Living. Breathing.
Living. Probably breathing
Lone being
Check
An intelligent being that can be reasoned with. Diplomacy should definitely be possible.
Well... That is the week point. Maybe he speaks at some point of some Saga, but he is not a diplomacy lover
Could control an entire realm/kingdom on their own by sheer force of will and personal power
He is expected to kill Thor during the Ragnarök. After which the few surving creature will reign on a new world!!!
Well... He is also expected to die during that fight but, hey.... It ain't over till the fat lady sings. Especially in Norse legends.
And... what about a twist that reveal your player are in a post-ragnarok world?