At level 20 you will be given a campaign quest titled "A Sliver of Darkness". This quest requires you to speak with Jucleas in Sanctum. Jucleas is the one that performed your Daeva Ceremony. If you forget where he is take a look at my map below.
After you speak with Jucleas and update your quest you will need to speak with Ludina. Ludina is at a pretty hard to get to location, The Outer Dock. In order to get to the Outer Dock you will need to head over to the Airship Dock in the middle of Sanctum. Below is a picture of where you will want to be standing while waiting for the boat to arrive.
Once the boat arrives, hop on it and enjoy the cruise to the Outer Dock. Once you reach the Outer Dock you will want to speak with Ludina the Bartender. After you've done that use your "Return" ability, since its the fastest way out of here. Now you will need to go to Veteron where you will need to speak with Morai.
Morai can be found south at the Miraju Holy Grounds. The fastest way to get there is to fly to Pilgrim's Respite and head west.
Once you're here you will want to speak with Morai and turn in your quest. When you turn in your quest she will port you to another region. Here you will find Icaronix, speak with him and he will fly you up to a cave on top of the mountain. Inside of this cave you will want to open Icaronix's Box.
The box will be located almost right in front of you when you land. When you open it, Ecus a ghost will spawn. Talk to him to get your very first Stigma and to open your Stigma Window.
What you will want to do is drag your Stigma into one of the Empty Stigma slots, both of them are marked in my picture. After you do that you will want to close the Stigma Window and speak with Ecus again. Now Ecus will summon an Assassin Spirit for you to fight. Kill the Spirit and you will be returned to Morai.
Update your quest again then Return to Sanctum. Back in Sanctum you will want to speak with Lavirintos to update your quest yet again.
Lavirintos can be found in the Protectors Hall. In the center of the Protectors Hall you will find an elevator. This elevator will take you up to where you can find Lavirintos. After you talk with Lavirintos you will need to finally speak with Miriya to turn in your quest. Miriya is right below where you found Lavirintos, still on the upper floor.