You will notice that in all towns (and not only) there are portals. You can access them only if you are a Seraph.

The portals are located in Lighthaven, Windhowl, Arakas Druids, Silversky, Raven Dust Oasis, Raven Dust Library, Stonecrest and Stoneheim Centaurs.

Approach a portal and click on it. Say Creations, Stop, Yes.

You step through the portal and appear somewhere else.

You have around 30-60 seconds to kill Doppelganger. Enter all 8 portals and kill the Doppelgangers.

You cannot go back into a portal you have beaten until you finish all eight.

Complete it and you will have access to Plane Walking (which means you can access the portals and travel to destinations at any time you want).

This map shows which portal travels you where.