Yume Nikki (loosely translated to "Dream Diary") is a freeware 2D top-down psychological-horror open-world adventure game developed and released digitally by Kikiyama for the PC on June 26, 2004. Along with minor updates throughout the years, it received a free Steam release by Playism on January 9, 2018.
Developed in RPG Maker 2003, the game follows a troubled social recluse (known as Madotsuki) as they explore the surreal (and sometimes horrifying) dream world while writing their progress in a dream journal (which is also used for saving the game). Along the way, Madotsuki collects special "Effects" that can change their appearance (most of which have unique effects, such as a frog humanoid form that improves their swimming speed) and encounters strange NPCs and events. The game's several surreal and disturbing sequences are never directly explained and are left open to interpretation (leading to numerous fan theories, including those of mental trauma).
Similar to other games built on exploration, the game features no "game over" sequence and no way to "lose". Even if an NPC teleports Madotsuki into an inescapable room, they can awaken back to reality by "pinching themselves". Although the game can be explored at a leisurely pace, finding all Effects grants a bonus endgame event.
Nearly ten years after the game's initial release, it received numerous official merchandise (including the game's novelization, an two-disk soundtrack with bonus arrangements, a vocaloid album, and a web-distributed manga adaptation). It later received a follow-up game, titled Yumenikki -Dream Diary-, on February 23, 2018.