They drop from 2 npc's. Comar Villard (49,11) is found on the right as you cross the bridge to the Wetlands. Easy to take down with the Voidwalker. Recommend heading to Hammerfall first and getting flight path.
Fighting Comar will set you to PvP. Good time to Hearthstone to Hammerfall, or to Undercity if you have completed the quest
Dalin Forgewright is close to Greymane Wall at 47,85 in Silverpine. Closest FP is the Sepulcher. Dalin is easy to spot as he is riding a horse. Make sure he is in the clear as you will die if others come to his defense.
Dalin can self-heal so you have to hit hard and fast. Once you have both hearts head to Undercity. Once you get the key item from the NPC you summon the Succubus