I completed the "good" ending of OMORI last night -- that game was a ride. (spoilers ahead)
OMORI is a game about grief, both dealing with it and failing to deal with it. Without revealing too much of the story, the game takes place partially in the real world, and partially in "Headspace", a bubbly imaginary world the protagonist has built to reside in and hide from their suppressed memories and emotions. In Headspace you and your troupe of friends explores and plays and has adventures with many cartoonish characters, while the real world mostly progresses the story and character development and is populated many of the same (less-idealized versions) residents as Headspace.
While most of the "action" (read: rpg battles and puzzles and quests) take place in Headspace, the events that happen there are for the most part meaningless. While they occasionally mirror things in the real world, it's mostly fluff. This is on purpose: the whole reason Headspace exists is to escape from reality. At the start of the game I found myself impatient to get back to Headspace where things were nice and my characters were leveled up, but by the end I was impatient to get finished with Headspace stuff to see what happened in the real world.
The transition between the two worlds, and the occasional incursion of one into the other, is there OMORI gets dark. The truth that the protagonist is desperately trying to bury surfaces, and he's forced to face his past and his fears. This manifests in legitimately dread-inducing visuals and audio that really elevates the game to the fame it's rightfully gained.
The game makes use of a handful of jump-scares throughout, but they don't feel cheap, usually leaving you uneasily thinking "what the fuck did I just see?". Married with the slower and more deliberate horror sequences of the protagonist literally fighting their fears, the game manages to frequently drive a cold knife into your gut.
On the topic of gameplay, OMORI manages to bring both of its charm and dread to its battle system. While it's a fairly simple turn-based combat system with cute or silly enemies, the game adds in emotion manipulation to the core battle strategy. You can make enemies and allies Happy, Angry, or Sad, with each emotion having pros and cons and being able to stack (eg. Angry->Furious). The emotions form a rock-paper-scissors triangle, meaning it's to your benefit to manipulate enemy and party emotions to maximize damage output.
While this mechanic is neat as a fighting strategy tool, it also adds a really novel way for storytelling within the otherwise simple battles. Boss fights often directly leverage this, swinging their moods in story-appropriate ways. It also adds potential to surprise the player with scripted or otherwise inaccessible emotions.
I don't have much to complain about with this one. Some of the quests in the real world felt fairly pointless (not an achievement hunter), and the longer side-quests seemed awkwardly placed at a point in the game where the story was at a tipping point which threw off the pace. Besides that this was a great experience with both bright and cheery highlights and gut-wrenching depths. It really sets you up then knocks you down with surprises right through to the end.