Fate/ Catastrophic Syncretism
This is a series of games made and run by Paorou ; which are in turn, spinoffs of the Fate/Stay Night Franchise, made by Type-Moon.
The games revolve around a ‘Grail War’, a secret war that occurs once every 60 years. It is fought in the shadows of Fuyuki city. Chosen humans, known as ‘Masters’, summon powerful heroes of man’s legend, called ‘Servants’, to aid them in their efforts. Only one wish can be granted by the Grail, and thus, they fight each other.
It is currently in the proofing stage, the first test campaign has been completed, called ‘Dirge of Fuyuki’. The second game, which has the accepted ‘system’ of play, is called True Dementia. This site will expand as more games are developed based on observations of the original.
The wiki is here. There is some more helpful info there.