This requires a premade group before entering. If you enter it by yourself, you will be in a private instance that no one else can join. It is recommended to have at least twelve players as even on normal difficulty it's still hard.
Here's a detailed guide on the full thing:
https://forums.elderscrollsonline.com/en/discussion/346563/n...It's located on the first small island in the cluster of islands on the right side of Vvardenfell. The first island being the one a part of the dotted trail. You can see it here on this map:
(Credit to
http://esomap.uesp.net)