Publisher Bethesda Softworks and developer Arkane Lyon announced Deathloop during the Bethesda Softworks E3 2019 press conference. Platforms were not announced.
Here is an overview of the game, via Bethesda Softworks:
Deathloop transports players to the lawless island of Blackreef in an eternal struggle between two extraordinary assassins. Explore stunning environments and meticulously designed levels in an immersive gameplay experience that lets you approach every situation any way you like. Hunt down targets all over the island in an effort to put an end to the cycle once and for all, and remember, if at first you don’t succeed… die, die again.