Return to the Soulless
Now that you have the white ember, make your way back to the Necromancer.
Use fast travel and head to his location.
Upon arrival though, the Necromancer isn't there and the pit in his den is now open. Descend into the pit.
Below, the necromancer lies in wait for you, going back on your deal and betrays you. This doesn't bode well... for him. Kill him.
The other enemies(Skeletons) are easily dispatched. The dangerous one is the ember Abomination. It can cast something under a specific character and instantly kill them. It can be seen in this picture under PrvtSC. If you see that white effect, move your character out of the way. The red circle that grows in the white circle serves as a timer.
When it reaches its largest form, it explodes and does big damage to a character.
Defeat the Necromancer and loot his corpse. You can find a unique wand that he was using.
At the back of the room you fought in, you can see the Lightbringer Heart. Take what is rightfully yours.
Fast Travel back to radiance.
Speak to Syrma on what to do with your heart.