Heretical Artifact is a written note found in The Knife of Dunwall.
Please log this superstitious artifact. It was found on one of Daud's men in a training area within the commerce building. There should be a crate in the temporary headquarters for items of this nature. Deposit it there after you create a log entry for it.
It can be found in the training center of the Assassins' Hideout during the mission, The Surge.