Put skeleton bones into Brazier of Solar at the Great Hessian Tree. AbyssSkeletons will appear. Defeat some of them. Then, speak to CaptainCromwell at the Great Hessian Tree. Use your quest tracker to see your progress.
Captain Cromwell: This should be enough to find out how much power is in Captain Cromwell: those bones. Excellent work, soldier. NakanoAzusa: What should we do now, sir? Captain Cromwell: Take them to the Brazier of Solar. Throw the bones in Captain Cromwell: there one by one. It'll reveal whether or not these bones Captain Cromwell: are from the Abyss or not. And if they are, we know it's Captain Cromwell: that bastard Sinclair.
Reminder Dialog
Captain Cromwell: Get ready for anything else thrown your way, soldier.