This is a storyline achievement and can't be missed if you do the dawnguard quest line.
This achievement is unlocked after leaving the soul cairn. You have to find Valerica, kill the keepers to free her and then defeat the dragon Durnehviir to retreive the elder scroll. The dragon has a special breath attack which summons undead to fight for him but apart from that it is a normal dragon fight.
You will gain the ability to summon him after he talks to you and this is required to gain the soul tear achievement.
You can also do side quests here and gain a mount from a quest to locate the horses skull.