This is a story related Achievement that can only be unlocked by the Tech-Priest Inquisitor.
Prophecy is a second campaign that unlocks after completing what is now called Season 1, which centers on the Martyr and the Alpha Pariah. The Tech-Priest Inquisitor has a unique story chapter as part of Prophecy and the Savant is encountered several missions into it. It's also a morality choice mission, with two separate opportunities for a morality choice.