The lair of the dragon Spite, largest offspring of the hellkite Malice, considered the last heir to the legendary Hatred of myth. The dragon was maimed by a previous invasion of its lair, one member of that group managing to blind it after downing it with a lucky throw. The fool's lance is still tangled in its torn, broken wings and his bones and shredded armor hang from Spite's horns like twigs tangled in the antlers of a deer.
Once filled with light reflecting on mountains of gold and jewels, the lair has gone dark, the coins melted by fire, precious stones ground into the caverns mud unnoticed. Statues and art torn to tags, weapons and scrolls rotting away. The gold no longer holds joy or fascination to the wyrm. Spite only guards it to keep it from others.
The fools trying to steal what remains of the dragon's horde (the players) aren't the only group who have crept in - a band of goblins, unarmored, carrying mainly slingshots, javelins and spears, are ever so carefully making their way to the heart of the lair, moving silently and covered in the wyrm's shed scales and spoor to disguise their own scent. They know their business and came fully prepared, and will not hesitate to draw the dragon's attention onto attackers with a thrown noisemaker. There are also miserable cave beasts, parasites, and shadows haunting the chambers, existing just beneath the wyrm's notice, and they will resist any disruption to the lair as a threat to their lives.
The dragon knows someone has come, and stalks constantly, screaming its hatred and frustration as it attempts to find the invaders. Even as terribly injured as Spite is, it's fierce, lethal, and cruel, hungry to repay the wounds inflicted on it 1000-fold to anything it can find. It would take a hero to finish it, and there are no heroes here, only greedy fools.
To draw Spite's attention for even a moment is to die. Any sound, smell, or vibration beyond the smallest will be pounced on. A clash of blades, a word of a spell, a cry of pain will call the wyrm in seconds, leading to a vicious, ugly end. You were stupid to come here. Only the utmost caution will get you out alive. Only unbelievable luck will let you leave rich.
Sharing this one here rather than rewriting it. I don't think this is a "dnd" kind of scenario. This is something more like Trophy Gold. Characters are going to die, and it's going to be ugly.