The King Black Dragon lair is a small dungeon located under the Lava Maze in the Wilderness. It can be accessed from the cage filled with lesser demons west of Lava Maze, and then by using the lever in the Lava Maze dungeon. It is the home of the strongest monster in RuneScape Classic, the King Black Dragon.
The dungeon itself is not in the Wilderness, only the entrance is. Therefore teleportation is allowed inside the lair, and players are safe from player killing once they make it down the ladder.