A pretty annoying achievement to obtain and some dedication is needed. The quest you need to be completing to achieve this is only available once a day so this is gonna take a while.
In order to unlock this you need to complete the in game achievement with the same name: Flawless sacrament. The conditions to unlock Flawless sacrament are to complete 6 other in game achievements. I will list them below with the method I used to unlock them.
First Sacrament: When you come into the dark brotherhood sanctuary there will an NPC in the cave like area called Speaker Terenus he will give you contracts to go kill NPCs in unique areas. These contracts are given only once a day. Simply complete your first one to unlock this.
Seventh Sacrament: Complete 7 of the above contracts
Fifteenth Sacrament: Complete 15 of the above contracts
Perfect Sacrament: When doing the contracts you are given one primary NPC to kill and several other optional tasks. In order to unlock this you need to complete the contract while completing all optional tasks. This needs to be done 5 times. I'm a magika based sorcerer so I didn't have any invisibility spells. I just took my time and was very cautious. Zooming out from your character as much as possible really helped me scout the playing field (hold down on dpad and toggle with right stick). Also, if you fail and get detected more than 5 times you can wait till your bounty clears, abandon quest and dashboard to completely reset your detection tries (you get 5 per contract). Once I came across invisibility potions I used them and it was extremely helpful.
Hidden Sacrament: You need to complete a contract with out a single detection. Not sure if you need to complete optional tasks or not but I did it anyways just to be sure. I got lucky and did this on my first try so I don't know if the dashboard trick from above will work.
Blackest Sacrament: The NPCs you kill have unique names, in order to unlock this you need to kill everyone listed in the in game achievement description. There are 21 NPCs listed in that list. It's completely random who you get each day but for the most part you don't get many repeats. In most cases you get to kill two in each contract which helps. However you will get repeats, you just have to stick with it and the names will continue to cycle through. I did it everyday so I don't know if that helped me or not.
When all 6 have been unlock you will unlock Flawless Sacrament and get your 20g. You also get a pretty cool memento that not many players have. Hope this proves useful and as always let me know if I can add/change anything.