You remember Durnehviir, the dragon from the "Beyond Death" quest? In your conversation with him as you leave the Soul Cairn, he gives you a shout to summon him in Tamriel. Unlock all three words of this shout.
Each time you summon him in Tamriel, he will teach you a word for a new shout called "Soul Tear." Summon him three times, and the achievement is yours.
Important Notes:
The shout must be aimed at the ground while charging, otherwise it will not summon him.
You need a large, open area to summon Durnehviir without interruption. Good locations include:
The windmills by Whiterun.
The area across the water from Harkon's castle.
Make sure to fully charge the shout to summon him.