This was basically included as part of Advent Children Complete. It tells the backstroy of Densel, the kid who randomly appears in Advent Children without much explanation.
Story: This basically fills in a lot of the blanks about Denzel, his relation to Cloud and Tifa, and the conditions of Edge. The story is pieced together with parts of both the game and Advent Children. It follows Denzel on his journey to discover someplace he can belong. The story is touching and a little sad.
Animation: The animation switches between CGI (like that in Advent Children) and regular animation. Both are done as one would expect from a company like Square, captivating and enjoyable.
Sound: The sound from Advent Children can be found in this OVA. If you liked AC's music, you'll like this.
Characters: Along with the group we already know, you are introduced to some new and interesting characters as Denzel moves around. Despite the amount of characters in such a short piece, they are excelently done and tie together a rather interesting storyline.