kigmatzomat
Legend
Just for sake of argument, let's assume 15th level characters. I want to spitball ideas on what the adventure looks like in the lead up.
All that said, 15th level characters will have lots of tools up their sleeve. I am sure there is plenty of magic that could potentially be used to locate the dragon and bamf into its lair, and I would like to make that harder. The dragon slept for half a millenia and had to be secure from dragon slayers and (more importantly) other dragons hoping to take its lair and hoard for their own. So the lair should be protected against scrying at least, if not also teleportation.
Forbiddance is a permanent spell that the dragon could have made a deal to acquire, or taken a lair belonging to another dragon that already had it. If you go Forbiddance, it took a month to cast so there's no reason there wouldn't also be an array of Glyphs of Warding cast by the cleric. Don't bother with the zot version, load them with spells triggered by command words or gestures.
Another defense to scrying is completely obscure the dragon's resting area. Opaque water, mist, vapors, fog, smoke, etc should be the norm when resting or under attack.
If the dragon is sufficiently crafty, it will create similar decoy environments elsewhere (i.e. a statue of a dragon in same obscurement) to get those handful of teleport errors. Fill it with monsters that don't use the dragon's element so any preparations are wasted. Add some kind of triggered Sending (maybe via a Glyph of Warding) to notify the dragon of someone entering a decoy.
I think Legend Lore would really only provide the same information the elves could give, because it has to be lore someone knew at some time.
Speak with dead or similar might work. Even a side quest to find a vampire or other elder undead., preferrably one with a known grudge against the dragon.