You will find Breaker Thram on the southern battlements of Skyhold. Speak with her to begin the quest ‘Way of the Reaver’. To complete the quest and unlock the Way of the Reaver skill tree, you will need to craft a Dragon Blood Distillation. This can be done at a requisition table once you have the following items:
Codex Entry: Writing on Reaver methods - Found nearby Iron Bull at Skyhold’s tavern, Herald’s Rest, or from the merchant Willvan in Val Royeaux
3x Infusion Primer - Obtained by killing 3 reavers at Crestwood, located in East Side Hills, The Black Fens, and The Flats. Can also be purchased from Saphi in Val Royeaux if The Short List perk is active
20x Rashvine - Found throughout Emerald Graves, Exalted Plains, and Emprise du Lion. Can also be purchased from merchants at Crestwood village, or from war table mission Exalted Plains Resources by selecting Josephine or Leliana
Note: You will need to complete the war table mission Specializations for the Inquisitor before you can find and speak with the trainer.