There is a balance in the universe and when that balance is upset all manner of evil can travel from one reality to ours. There is nothing that upsets that balance more than the death of a good and holy soul.
That is when the Warriors go to work.
They have existed since the dawn of human kind. Part human part alien they are dedicated to the protection of this thing we call reality. They are here to defend us against the monsters we cannot even see.
But there’s a problem this time. The monsters are getting help. If the Warriors cannot solve the problem of who and how in time everyone will be able to see the monsters and the world itself will be a half step away from doom.