Don't know where to begin?

While ultimately it doesn't matter which arc you begin with, Cecil arc is recommended if you want my recommendation.

And then Masu arc + Shattered union arc. 

Feel free to visit the lore section as well which holds stories of its own.

The blue planet arc (2)

This arc should be read after reading alien arc, but it's standalone.

[The blue planet arc] [1] [Awakening] [10300]

The blue planet arc comes after the alien arc. This arc represents the last part (era) of Two Clusters.

[The blue planet arc] [2] [Deep blue] [17866]

This is essentially the last story of Two Clusters due to it being the end of Cecil. But it does not mean it is the actual last story of the story. I do not have the recorded date of the creation of this story. However, this was one of the first few stories I came up with when I was starting to write Two Clusters.

Meaning that the concept of this story exited in as early as 1990 and I release this story in 2019. It has taken me 30 years to get the stories advance far enough for me to be able to release this.

Again, this is not the end of Two Clusters. If anything, this is a new beginning of Two Clusters.