The Elder Scrolls V: Skyrim Special Edition

Pressing ~ will open the console command window. (top left under ESC)

You want to type into the console:

setstage DLC1VQ05 200

Your achievement should pop.

THIS HAS MERELY BEEN COPIED AND PASTED, ALL CREDIT GOES TO Madlmax120 for posting it back in 2012. Unsure why it's empty for the SE verison as I'd have thought someone at TA would have copy pasted. Non the less this will shave about five seconds of having to type the achievement into google.

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.