Today Microsoft has publicly announced Bethesda and Zenimax have joined Xbox. In the blogpost by Phil Spencer, the arrival of Bethesda games to Xbox Game Pass has been confirmed. The Evil Within seems to be one of these games hitting the service this week. The PC version on the Microsoft Store was found today, with a release date of March 11th.
Other Bethesda games are also expected to release on the service now the acquisition has been finalized. Doom 2016, The Evil Within 2, the Wolfenstein games and The Elder Scrolls: Obsidian seem like logical candidates.