Daobeth

Daobeth was a kingdom that was the most powerful of the five kingdoms, until it was defeated by Dragons.

History
Daobeth had the strongest citadel out of the five kingdoms, the kindom was eventually defeated by dragons. From then on, it was only ruins.

When Merlin travelled to the Isle of the Blessed, he went past Daobeth, as it was seen when he was travelling (Le Morte d'Arthur).

When the veil between the two worlds was torn, the Dorocha were released, Arthur intended to travel to the Isle of the Blessed to repair it. On the way, they went past Daobeth and spent the night there, thinking they'd be safe from the Dorocha. But they thought wrong. The Dorocha broke in and harmed Merlin, who was protecting Arthur.

Arthur later sent Lancelot to bring Merlin to Gaius so there's a hope of saving him. After that, Arthur and the Knights of the Round Table departed Daobeth and continued their journey in vanquishing the Dorocha (The Darkest Hour).

Trivia

 * The only time the kingdom was identified as "Daobeth" was in a deleted scene in The Darkest Hour: Part One, when Arthur was saying that it was one of the strongest kingdoms.