. . . in Lindisfarne, towards the end of a narrow ravine in the far NW of the territory. To multiloot the black chests in there (there are two of them) you need to set a Lloyd's Beacon sufficiently far inside to be past the point at which you trigger the appearance of the dragon.
IIRC, what you need to do is first kill the dragon, then set your Beacon, then leave the cave and save your game outside. Then merely Beacon back inside, take the treasure, and leave normally. You will hear the dragon arriving as you make your way to the exit. Repeat as required – the cave respawns every time you re-enter it, and since you trigger the dragon's arrival as you are leaving rather than entering, it never catches up with you, unless you're very dilatory about leaving!