Archdragon Peak. Spoiler tells how to get there. [spoiler]after beating Oceiros, there's a chest with a secret wall behind it. This leads to the Untended Graves, which leads to what I personally think is the hardest boss; Chamion Gundyr. Anyhow, inside the Untended Graves you can find the Path of the Dragon gesture. Use this Gesture in Irithyll Dungeons, near the Dragon statue, on the cliffside, and you'll be brought to Archdragon Peak, home of the Nameless King.[/spoiler]

Archdragon peak secret area