The entrance to Thais Dragon Lair is situated north from Thais on third level under ground (here). Although the route is a little bit complicated you probably won't encounter any creatures apart from Trolls and Rotworms. The route is explained in Naginata Quest spoiling information. If you're the owner of dark mansion this is a great hunting ground for you, since it's easily accessible from the basement so you can make a quick trip to the house to drop loot and gather supplies.
In the Thais Dragon Lair you can find 3 Dragon Lords (one spawns near the center of the cave, this one spawns REALLY fast, and two in the quest room), 2 Dragon Lord Hatchlings spawning in the northeast corner and 3 Dragons and 4 Dragon Hatchlings spawning all over the room. The dragon lord boss Demodras may rarely spawn as well.
It's a good hunting place for mid-level Free account players.