The showdown an entire series in the making...
E6 is essentially the "after story" of E4. Where E7 tells the after stories of E1/2/3/5, E6 serves this purpose for E4 while also leading directly into E1. Therefore, neither E4 nor E6 have any portion in E7. This installment brings the series full circle, before E7 ushers in its climax and resolution.
For a long time, like E4, E6 remained under-developed compared to the rest. It was with the inclusion of the Dream World that it rapidly expanded and gained its own identity amongst the others. Also like E4, its story is split into "chapters" focusing on small groups of characters that converge when they all arrive at the Dream Shrine.
Per the tagline, this refers to Luminaya VS Cugei... E6 encapsulates the "Dark Flood" spoken of in E1, setting the stage for the original.