Marked for Greatness - 30gs
Seal 75 rifts in a single playthrough.
There is noting wrong with cheeseninja123's solution, and all of his tips are going to help you complete the achievement. But I thought this would come in handy too for anyone working on this achievement.
First, fade rifts are marked on your maps as a Black Flag with a star cut out of the middle. If you select the fade rift as your primary quest, it will place an Inquisition eye marker over each rift associated with that quest. In MOST areas, the fade rifts are spread across multiple quests. One quest may require you to close 2 rifts in one area of the region, while another may require you to close 4 rifts in a different area. Make sure you do all of the quests!
Second, here is how many fade rifts are in each area, listed in the order of area difficulty/required level, AND how many quests the rifts are spread across:
Hinterlands - 12 (4 quests)
Storm Coast - 3 (2 quests)
Fallow Mire - 2 (1 quest)
Crestwood - 4 (2 quests)
Forbidden Oasis - 5 (2 quests)
Western Approach - 8 (4 quests)
Exalted Plains - 11 (2 quests?)
Emerald Graves - 11 (4 quests)
Emprise du Lion - 6 (3 quests)
Hissing Wastes - 12 (3 quests)
You will notice if you add all of these up, it only comes to 74 while the achievement requires to get 75. Well, any rifts that you close via the main story line or side quests are also included in this achievement, so that is how you obtain 75+ closings. So you must progress the story or do side quests to ensure that you can get this achievement.
This achievement will also help you with:
As all of the enemies that spawn from fade rifts are demons.
Please comment before feeling the need to downvote. I cannot correct my solution without knowing the issue!
Good luck and happy hunting!