It's Always Sinny in Revendreth

You must finish the Revendreth main storyline before you can begin working toward this achievement, as it unlocks access to the Sinstone system and Archivist Fane.

Head to Archivist Fane at 72.4, 49.4 inside the building in Revendreth. He sells Inquisitor Sinstone, High Inquisitor Sinstone, and Grand Inquisitor Sinstone for 100 Sinstone Fragments each. He will only offer a Sinstone for sale when you have no other uncommon (green) Sinstones in your bag, so use each one before buying the next.

Some Sinstones can drop from specific mobs in the world — for example, Inquisitor Otilia's Sinstone drops from the Depraved Interrogator, which is a rare spawn. Purchasing from Archivist Fane at 72.4, 49.4 using Sinstone Fragments is a more reliable method than farming rare mob drops for any Sinstones you are missing.

Use each Sinstone to summon and defeat the corresponding inquisitor. You can still receive credit for the kill if another player uses a Sinstone and summons the inquisitor, as long as you tag the mob before it dies. The Sinstones purchased from Archivist Fane are awarded randomly, so buy and use them one at a time at 72.4, 49.4 until you have defeated all required inquisitors.