Normally the weird monster has the advantage on it's home turf. But if you want to throw a CR11 encounter at a lvl6 party, you're going to want to give them an edge.
First, I'd forgo any NPC's; running a dragon is enough.
For the dragon itself, I'd tweak some stats here and there. Look at some CR9 monsters and their attack bonuses, saves, DPR and DC's. That breath weapon looks like a TPK to me, as-is.
Now, the important part: the actual encounter. I'm imagining...crumbling ruins alongside natural tunnel systems? I'd map out some areas where the dragon would need to squeeze and some where the characters would need to and the dragon couldn't get to at all.
You could dust off the cave-in rules so the PC's can't just stay holed up forever and give them the means to trap/injure/finish off the beast.
Add in...some bats or shriekers (to help them hide from the sightless dragon's other senses), some old traps they can try to use against it, and...I don't suppose they can get suited up beforehand, with a few last-minute consumables or whatever? Potions of Bear's Endurance, Align Weapon, Darkvision, etc. A dragonbane weapon might be a little too Zelda-y, but something like that.
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It could be a really dumb dragon by dragon standards. And that is why it was fooled and looked up as an guardian.
Dragons are mostly deadly because they are super intelligent and strong, and have a devastating breathweapon and cast spells and live long and have multiple plans and allies.
If this specimen are hatched dumb, then you would really give your players an edge.
As Khan says, how you play a monster and use their abilities can have as much impact on the true difficulty of the encounter as any mechanical changes. Dragons especially have personalities and individual quirks, being intelligent and sapient. Maybe this one is unintelligent and not good with spellwork; maybe it ran afoul of a powerful witch and was on the bad end of a bestow curse spell that renders it unable to use its breath weapon. Even pride can weaken it, making it think the party too incapable to be worth smarter tactics like staying off the ground or using terrain to its advantage.
An NPC can be a good way to give some tailored support if you think they need it, although as was previously mentioned you'll have a lot on your hands already without having to run one. A cache of potions/oils like align weapon or protection from energy gifted to them would serve much the same purpose without creating more burden on yourself. A spectacular source of information they can use to figure out their foe, such as a hidden library or a diviner NPC, could also strengthen them a great deal while giving them more agency in their preparation, since they're the ones seeking out that information.
Playing a monster true to it's nature is important. A normal bear isn't going to charge headlong into combat, maybe even after it's been attacked once.
But all this talk about dumb dragons and effectively pulling punches doesn't sit very well with me. Better to just present a more appropriate encounter than to pretend at a super-hard one and coddle your players. Design the encounter fairly, play it ruthlessly.