For base outfit: 

You need acces to Npc
(server must have killed Gaz'haragoth on the central mountain)
You have to finish 4 missions in Roshamuul Quest
- Mend What's Broken
- A Cup Of Tea
- Ancient Dreams
- Waters Of Life
Talk with Npc
Hi/Outfit/Yes/Base
For first addon you need: 

Dream Warden Mask (lootable from: Gaz'Haragoth, Horadron, Omrafir, Prince Drazzak, Terofar, Zavarash)
Talk with Npc
Hi/Outfit/Yes/Mask
For secound addon (you can only do this addon after first addon) you need: 

Dream Warden Claw (you can get it by using Unrealized Dream
)
Talk with Npc
Hi/Outfit/Yes/Claw
For full outfit you will gain achievement "Dream Warden"