If you read the dialogues it should be pretty clear. For instance Cydonie will direct you to the giant in GH who will tell you about how you need to run the 3 quests and collect the 20 relics. Then once you're done she'll tell you about the dragons to kill in the Tor. In the Vale you first get the quest to reach Meridia, then the NPC in front of Shroud will tell you about how you need to go find the Twelve members (and when you go back to him without having done all 5 quests he'll tell you you need more).
The quest logs aren't always complete unfortunately, and sometimes if you missed the NPC dialogue you can't really get the full info anymore. I don't remember for sure what it says about Reaver/Shroud though.I'll have to check my quest logs though, does the raid show up and indicate what I've done and haven't done to gain access to it?