The Spirit Realm

Yes, you need an expedition to get into the zone. It's an instanced zone, there is no static Anguish.

When you do an Anguish run, when the aug droppers spawn you get a 2 hour timer. I believe that timer is just called "Anguish, the Fallen Palace" with no event name. When you kill a raid mob in there (the ones that drop gimp keys/loot), you get a timer of somewhere around 3 days. As long as you don't have the 2 hour timer, you can get the instance.

If you join the open raid, you will have lockouts on any mobs that raid kills. That means you won't be able to join your guild for those, so it'd be a bad idea.