Maybe you could use the stats of Empress Donata (ECG p.98).
that's definitely a good start, but i also want to make him a badass in melee. suggestions?
Using the Monster Builder makes it easy to custom Empress Donata as the KoD:
Monster design in 4e is actually kinda fun, because it's a lot easier to get your concept of how you want to fight to go right in the mechanics.
So that's where you should go first. You already mentioned you want him to be a badass in melee, and that can be done in a variety of monster rolls, excepting Artillery.
The first question you should ask, is how much "stage presence" you want the King of Diamonds to have in the climactic fight. Do you want him leading an cadre of bodyguards? Maybe he's a little bit tougher than each of them, a few levels higher, but isn't much more or less of a participant in the battle than them. Maybe most of his "threat" is in empowering his allies. If you like this idea, Empress Donata's stats work very well.
But maybe you want a much bigger spotlight on the King of Diamonds than any other participant of the battle. Do you want him to be twice as tough as his allies? Do you want him to wade into battle, headless of attacks of opportunity and Marking effects because he's got HP to spare? Make him Eltie.
Do you want to go even further? Do you want the entire climatic fight to be about HIM? Do you want him to have abilities to counter or mitigate several of the PC's tactics? Do you want his allies in the battle, if there are even any, to be ultimately meaningless and overshadowed by his indominable prescence? Then make him a Solo.
Then from there you think about the fight. If you were writing a book, what cool things would the King of Diamonds do to make the readers realize he wasn't just some smuck with a sword? You already mentioned one of those, with the turning into living diamond thing. Maybe the King of Diamonds has an Immediate Interrupt ability where he gains Resist 10 all until the end of his next turn, triggered by being attacked in melee? Or maybe it's a free action when he hits bloodied, he just turns to living diamond until the end of the encounter.
This is where you brainstorm abilities. So you mention you want him to be a badass in melee. How exactly? Do you want him to make huge sweeping attacks? Give him Cleave and a close burst 1 recharge attack. Do you want him to make devestating single-target blows? Give him an at-will attack that imposes an AC penalty on the target when it hits, and a recharge attack with crazy high damage.
So yeah. Go ahead and think of the battle in your head as if it were a story. Plan out his super-moves and general fighting style. Lemme know what you come up with, and I'll help you put it into stats.