If you follow the quest line from dawnguard you come by a quest that you must go to the soul world.
At the end of the quest in the soul world you get a shout that you can call a dragon ( Summon Durnehviir).
You must unlock this shout by learning the 3 words and unlock them with 3 dragon souls.
Then go back to skyrim go to an open place and call the dragon 3 times with the shout (Summon Durnehviir) (you must shout on the ground to call the dragon) and you get the trophy.
After that you can unlock soul tear with 3 dragon souls.