Sony declared the following games will be free to PS+ members in December 2018.
SOMA, PS4
Onrush, PS4
Steredenn, PS3
Steinsgate, PS3
Iconoclasts, PS Vita (Cross Buy with PS4)
Papers, Please, PS Vita
All games will be available from December 4, 2018 to January 1, 2019.
The Warframe: PlayStation Plus Booster Pack III is also free and includes 100 Platinum, 100,000 Credits, Quanta (a Corpus laser rifle), Quanta Obsidian Skin, 7-Day Affinity Booster, and a 7-Day Credit Booster. PS Plus members can get the pack for free from now until January 15, 2019.
