Ancalagon the Black is an old dragon, set in his ways. He's lived in his lair for a long time, in fact, he was responsible for it being a ruin. Over 500 years ago, he flew in from the west and slew the men and elves of Corvel on the Mount in a devastating early morning attack. None survived and he immediately moved in and started gathering his hoard.
A few years ago, a clutch of basilisks moved into the ruins and Ancalagon was surprised that he could speak with them and wasn't affected by their gaze. The basilisks have a free rein in the ruins and make excellent watch-dogs (and rather final security systems).
History of the name: Ancalagon is a dragon bred by Morgoth during the First Age to be the mightiest, from the Middle Earth books by J. R. R. Tolkien. He was slain by Eärendil, who destroyed the dragon by casting him onto the triple-peaked towers of Thangorodrim.
Ancalagon the Black, old Black Dragon, Age: ~800, Desolation: Bymarsh, Lair: Ruins of Corvel on the Mount, Armor Class: 2 , Hit Dice: 7, Hit Points: 35, Attacks: claw/claw/bite (1d6/1d6/1d6), Special: Spits acid (35 damage, area 60ft by 5ft), Move: 12 (24 when flying), Spells (3/2/1): Sleep, Shield