You will find Viuus Anaxas near the front gate of Skyhold. Speak with him to begin the quest ‘Way of the Necromancer’. To complete the quest and unlock the Way of the Necromancer skill tree, you will need to craft a Ceremonial Mortalitasi Skull. This can be done at a requisition table once you have the following items:
Codex Entry: Writing on Necromancy - Found on the top floor of Skyhold library, near to where Dorian can be found sitting. Alternatively purchased from the merchant Willvan in Val Royeaux
3x Nevarran Skull - These can be found from skeleton heaps at the Storm Coast (one south of the the northern camp, next to the astrarium south of the Hessarian Camp, and atop a hill beside the river south of Hessarian Camp. Can also be purchased from merchants in Crestwood if The Short List perk is active
10x Bloodstone - Found throughout Emerald Graves and Emprise du Lion. Can also be obtained from war table mission Emprise du Lion Resources by selecting Josephine or Cullen
Note: You will need to complete the war table mission Specializations for the Inquisitor before you can find and speak with the trainer.