Has anybody mentioned the Door To Darkness?

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This collection has 2 games and a movie.

Kingdom Hearts 3D: Dream Drop Distance

This game is focused on the sleeping worlds and the mark of mastery exam. Very important game to the story, has cool worlds, the drop mechanics could have been annoying at times, getting the "drop me not" ítem helped.

Kingdom Hearts 0.2: Birth by Sleep – A Fragmentary Passage

Its like a 2-4hr long game. Extremly short and it serves as a way to set the scene for kh 3.

Kingdom Hearts χ Back Cover,
It expands more on the events of union x mobile game which isnt available anymore. I watched union x cutscenes on yt and then back cover. It overall sets an important scene for kh 3 and ties everything together. Its rly important to the story.

This collection contains 1 game, 1 demo and 1 movie so I will break down each.

Dream Drop Distance:

Originally a 3DS game which has been ported over well. It also introduces the flow motion system which appears again in KH3 which allows for faster and more dynamic combat. Overall it is a cute game that serves as a nice transition game between KH2 and 3.

Birth by Sleep 0.2:

This is a 30 minute experience which covers Aqua from the end of Birth by Sleep to KH1. It definitely feels more like a demo, especially cause they start you at level 50ish though this works with the narrative. It more is just a mini episode showing what Aqua is up to during this time and gives you an idea of how KH3 will look and feel.

X Back Cover:

The original KHX was I believe a browser based MMO with this covering the story of the masters who would go on to fight in the first keyblade war. It isn't absolutely vital going into 3 but it is a cutscene compilation so no reason not to give it a watch. It helps flesh out the world and give some clues at things we will see in the newer installments.

Overall this bundle was fun though a bit short. 3D is definitely my favorite of the bunch as it feels more like a full game but the other two are also fun/interesting.