Hmm, this certainly has potential but a few questions should be considered first of all. If this is like the Fate version of summoning then how would command seals work? After all, command seals do give control over a Servant and binds them to their Master. If a Master told their Servant to have no mercy against the others then, should a battle happen, the Servant of said Master will not stop to consider letting his/her opponent survive. Also, what about classes? Do they exist? How will magic work in this world? Finally, would there be an equivalent of Noble Phantasms in  this world?