Opportunist

This is anAdvanced class artifacts: these look like advanced tomes and go in the Grimoire slot. They are not part of the Rune/Scroll/Jewel/and Relic four-piece artifact sets; the game files them as grimoire equipment, which is why you will not find them under the Artifact category.
Where to get them: from the Box of Mastery (one random advanced class artifact); some can also be obtained at that class's Master by fusing one of each of the four pieces from the same artifact set.