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Yume Nikki is a freeware indie horror game made in the RPGMaker 2003 game engine. It's a unique game in that it has no dialogue and most of the game's meaning is up to the player's interpretation. In the game, you play as a dreaming girl named Madotsuki who travels through surreal dreamworlds looking for "effects". Each one change's Madotsuki's appearance in some way or gives her an item to use. Some effects are entirely cosmetic, while others can be used to interact with the environment or NPCs in a new way. When the player collects all 24 effects, they can access the game's ending.
Much of the Yume Nikki experience is exploring the dreamworlds and taking in all the bizarre, moody ambiance the game has to offer. It's not just about collecting the 24 effects; there are a lot of optional areas and game events--many of which have a random chance of occurring--that are even more interesting and compelling to check out. The game can be confusing to play without a guide, especially because many of the maps are large, somewhat empty, and looping (meaning, if you go far enough in one direction, you'll end up right back where you started). The game is definitely not for everyone--especially with the horror elements and lack of concrete answers to, well, anything about Madotsuki's life and experiences. But for fans, it's something really special.
Yume Nikki has inspired dozens of people to make their own fangames that draw upon the style, setting, and game mechanics that Yume Nikki established (player must go to sleep, explore dream worlds, and collect effects, no dialogue, etc.). YNFGs--Yume Nikki fangames! I really like that many of them use Yume Nikki's elements in different ways. For example, the YNFG .flow has smaller, more navigable game maps to make for more effective environmental storytelling. Also, there are more recurring characters and events that elaborate upon the main character's life experiences. I feel really invested in the story it alludes to, and really impressed at what it can show me (and make me feel) without dialogue or traditional game mechanics. It feels a bit more cohesive than Yume Nikki is, to me, which I really appreciate. On the flipside, Yume 2kki is another YNFG that's very different; it's much less cohesive than Yume Nikki because it has a TON of maps made by lots of different people! It's collaborative, big, a little messy, but far from boring! Check out my YNFG fanpage for more.
The easiest ways to play it on a computer nowadays are either downloading it off Steam or playing the multiplayer browser version, Yume Nikki Online (which might work on mobile devices as well, I haven't tried it yet). Both options are completely free. Historically, you'd download the game from somewhere like a fan wiki and you may need to download an English patch as well as an RTP (runtime package for RPGMaker--additional files for the game engine). And then you might need to set your computer's locale to Japan using AppLocale. Running Yume Nikki this way, or running most YNFGs, nowadays we also have a great tool called EasyRPG that saves a lot of trouble.
The creator of Yume Nikki, who goes by KIKIYAMA, is rather secretive and quiet, but a few Yume Nikki-affiliated projects and the like have happened over the years.
Fanworks specific to Yume Nikki--not listing fangames here (unless they're specifically about Madotsuki and her dream world, like Yume Nikki 3D).